Paleo is intentionally low-carb because as a diet, we choose to use fat as our main source of energy as opposed to carbs.

I disagree that this is a cornerstone of "paleo". At least, that the reasoning itself is (even if the end result is similar). Many paleos do, and it's a popular route to take, but I can't agree that that way is some kind of "one true paleo". Paleo can be high-carb (that is, relative to other paleo-eating-people, not necessarily relative to the world at large) just as easily as it can be low-carb. Some people combine paleo and keto to get the kind of diet that you're describing, but it isn't mandatory.

However, the main problem I have with this particular user is that he isn't trying to be helpful. He's just trying to get everyone to eat more carbs, whether it is for their benefit or not, because that is his prerogative

I worry that you (and I, to a certain extent) feel this way because the user in question doesn't agree with us. If he actually believes what he's posting (which, really, I haven't seen any indication that he doesn't), then of course he's trying to be helpful, just as you or I would be trying to be helpful if we were posting in some other sub about this great diet that we've found, even if everyone in that sub didn't agree with us (though, I think you or I would eventually shut the hell up if we were constantly getting downvoted to invisibility).

I do see your harassment point, though, and it's a good one. Regardless of whether he believes it or not, he's definitely a bit pushy with it. The harassment angle is an interesting one that should be discussed further. At what point does something qualify? Personally, I'd say as soon as the user they're responding to feels harassed (and hits the report button, 'cuz otherwise, how would we know?).

Regarding the flair: it would be more of a warning label. Like "this person does not really represent the views of the paleo community at large". They'd still be free to post, but with a warning that "hey, this person probably isn't the best person to take advice from." (not every comment on here is advice).