How do you convince someone that supernatural things don't exist?

You can't change someone's mindset easily. There is no simple red pill which will make someone agree with you. Instead it's many small steps.

astrology signs

This one is quite easy to explain if you know basic astronomy. Just use some program like Stallarium and explain that the constellations don't match with the date, but they do if you turn back the time for a few hundred years.

Whenever I tell them that science has produced no evidence of any of these things, ...

I have a big problem with this. People claim to like science, but almost nobody has any fucking clue about science, what it is, or how it's done. Science isn't being like, "I wonder what ammonia and bleach make when combined" then doing it in front of a camera--that sort of shit is just entertainment, stupidity. Science is boring as shit, all about long and drawn-out processes designed to be as thorough as possible.

I say that to say this: there was no evidence of anything until there was. What we know now isn't the extent to what can be known. Later discoveries will be made which will topple previously held beliefs. We may believe it as fact now that there is a female penis, but eventually science may determine that there is, in fact, no female penis at all and that it's just a man in drag; however, as of now, science has produced no evidence of any of these things.

Worry about yourself.

You appear to be a strong analytical type. You have to remember that these people aren't argued into what they believe ... you probably were so talking to them is likely to be very frustrating and you will feel they are being dishonest (which they are, but they don't view truth as you do). I suspect you would choose to know the truth even if you discovered something horrible, such as your life had no meaning. Most religious people are scared to death of that.

I think the root of the matter is you have to discuss faith (pretending to know things you don't know) and bring attention to it as an unreliable epistemology. Well maybe it could be true ... well that's true of everything, SDBH mods could all be rothschild family members planning to take over the world too ... etc.

I've mostly not tried to help these types over the last few years, I think they know they are pretending, if it were any more than an emotional need they'd have spent a good 45min researching the subject (and I can guarantee you your friend here didn't go to the library and say "Yes I'm really superstitious, I was hoping to find a book by an atheist who used to share my beliefs but no longer does, probably with lots of credentials and experience". However, that is the absolute first thing anyone on an honest quest would do, and that's EXACTLY what you would do though you probably already did it (or maybe figured it out yourself, I think you said elsewhere you'd been in third world countries you can see how humans think).

Oh if they are religious and pentecostal you could show them Marjoe.

Look up the Invisible Pink Unicorn. It targets this exact thing.

https://en.wikipedia.org/wiki/Invisible_Pink_Unicorn

Of course there are frauds and hucksters.

There are also real effects that have incorrect explanations

A lot of these things are placebo effect, for example. This does not mean you dismiss it as "only a placebo effect" Placebo effects can be impressively real. One spectacular example is this medical case.

Incurable Skin Disease Completely Healed By Power Of Suggestion

https://www.youtube.com/watch?v=V22CTJwQkjM

so if you dismiss this out of hand, you can be going against what they "know is true" even if their explanation is obviously wrong.

This case also whistles by (i.e ignoring) the implications of "the power of faith" --- Placebo results from the fact of believing something is a certain way.

You are also dealing with fixed mental models of the universe. This can get rather religious in weird ways. Sort of like people with stubborn political beliefs and prejudgments of situations and people.

So when you say these things are stupid, you are coming in at too high a level. You need to address the problems at a much more fundamental level.

For example

Racism is basically wrong because there are 48 visually distinguishable human types, and skin color does not really correspond to other characteristics like blood type, etc. It is entirely superficial. There is no blood test for whiteness.

see Artist's Guide to Human EthnoTypes

http://cedarseed.com/portfolio/human-types/

and the Nazi version of racism is event more superficial, because it is entirely tribal (german race, british race, spanish race, etc)

There are many human types. But the old ideas of race are obsolete and laughable, because the reality of the human condition is sooooo much more complicated.

What they should research are the genetics of their own personal ancestry.

Origins, spread and ethnic association of European haplogroups and subclades

https://www.eupedia.com/europe/origins_haplogroups_europe.shtml

also

https://www.familytreedna.com/projects.aspx

for example

https://www.familytreedna.com/groups/r-1a/about/background

quick chart http://i.imgur.com/IKLXss5.jpg

get expert on this, and a lot of other weird ideas fade away.

so the bottom line is saw away at the foundations of their unfounded belief with things they can get interested in.

Are you speaking about yourself in third person?

You cannot reason someone out of something he or she was not reasoned into.

Believing in something that doesn't exist is essentially wishful thinking. The problem is, of course, when it crosses from something that you wish were true to something that you believe to be true. And believing in something that does not exist is delusional which in adults is a mental illness. Yup, if you think about all those goddos out there as mentally ill it does rather put a different perspective on things.

Anyhow, argue with a mentally ill person? Good luck with that. Consider yourself ahead and just move on.

Look into street epistemology, its exactly what you're asking for.

https://www.youtube.com/user/magnabosco210