1) Banning their IP range doesn't do anything whatsoever, don't be a tard. Proxy, tor, etc... forget it.
2) Probably not possible, ToS... and heck knows he'd just blame it on some kid being on his computer.
3) See 2, also prevents the possiblity of any future sales, and since it would prevent him from using the games he bought, it would be illegal.
4) See 1. Ban one IP address, a whole range, a whole country, it doesn't matter, you just bounce your connection someplace else. It can be done indefinitely - until he gets bored, which can just be accomplished by ignoring him in the first place.
Right... if Ironclad hasn't thought to have /ignore commands be usable on a name in a list or temp-banning people who join/exit/join/exit or otherwise flood, then yeah that's pretty silly. A 5 minute ban would be adequate in almost all cases.
Even so, just ignore it, he'll go away when he notices something shiny.