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".
I don't think it'll help much.
There are plenty of dictionaries that define it, albeit mostly regarding mass-email advertising. But there are a few decent dictionaries—UrbanDictionary or the Jargon File—that cover more colloquial usage nicely.
Subtle
Frankly, I don't think unintentional spam is a real issue on any but the largest social media sites. Anyone too Down-syndrome to understand that their behavior violates good netiquette never would have found phuks in the first place. Playing dumb is far more likely, and making the TOS more wordy won't really fix that sort of trolling.