Again, I don't know why this bothers people so much. It's banter. I've joked about Utd fans all being from London on here plenty of times (and no, I'm not from Liverpool).
Fair enough, it might annoy some people but it's not a big deal. As said, if it annoys you, ignore him.
Also, the fact that he doesn't aim his posts at individuals and instead at Utd (for example), surely makes it less offensive. It's not a crime to dislike Utd.
If it doesn't bother you (not you specifically) then don't complain about him. I've found some of cheets' "arguments" with Utd fans funny because it's been friendly banter (or seemed that way anyway), it's only when it becomes personal and the snide comments come out that it starts damaging the forum.
Maybe cheets can tone it down slighty but he can't argue with himself so if you (again, not you personally) take exception to what he says, ignore him. If it doesn't bother you then throw a bit of **** (in a friendly way of course) back.