Alright, I would like to preface this by saying that I've never been banned by TF2C. This is just something that occurred to me.
Basically the revision that I have in mind is this: I think the only people who should be banned from tf2center should be hackers and maybe map exploiters. The reason I say this is because tf2center already has a !report command to remove an individual from a lobby. Therefore, griefers are easily dealt with. A possible solution to repeat griefers would be a function that bans a person from tf2center for, say, 3 weeks if they are !report'ed, say, 5 consecutive times, or 5 times within a span of time.
Now, most of the griefers are server owners who are activating sv_cheats or ending lobbies early because they're mad. I would then suggest a lobby-creating whitelist as opposed to full banning from the service. This could be accomplished by, for instance, implementing a server feedback sort of thing where people have the option to report a lobby post-game. If a certain percentage of people report a lobby, the lobby leader is barred from creating any new lobbies. This could also be based on consecutive times, or amount of situations in a time-span.
I do not support griefing, I just think banning should be reserved for hackers and maybe exploiters. If you have any questions, or want me to further elaborate on anything, I'm available.