It seems obvious right?
But it's not. It is a slang term that has no hard definition. It gets used very differently by different people.
I think most people more or less agree on the definition, but there are outliers who need it spelled out for them.
By posting a definition in the TOS or elsewhere, it could shut-down some unintentional spamming, and also makes the trolling a bit harder.
In past I have used this: "Spam being defined as irrelevant posts, chronic reposting, or advertising"
But due to some extreme retardation in past, I would add "Spam can take any form of message. Comments, posts, private messages, phone calls, smoke-signals, or other".
People can post those if its on topic for the sub they're posting in. I do remove some of the legit spam posts and banned one from a system sub the other day, can't remember which. Sometimes I leave the posts so the account is downvoted, making it harder for them to meet requirements to start their own spam sub.