Maybe make something like a thread or tribe specifically for those people that want to sit there and gain imaginary exp?

I think someone already created that (not going to link it to discourage its use), but as was pointed out to them while it may get one's XP up, it may be harmful to one's reputation. And reputation again affects not only how quickly you earn XP, if it gets low enough I believe you're temporarily barred from posting.

The Voting FAQ also reccomends:

Users should down vote comments that are irrelevant to the discussion or obvious attempts at spamming or trolling.

So potentially these low effort comments could be counted as spam, though then it becomes a matter of intent. E.g. welcoming someone to Snapzu (as noted by sushmonster) probably isn't spamming even though the comment may only contain one word as it's a more concrete act of welcoming someone into the community. Something akin to just posting "Cool!" might be considered spam though, as a simple upvote may be more appropriate in that case.