Its like this, there are actually only a very small number of BZ2 griefers, but they are quite experienced at hiding their true regular player identity, they often impersonate other players, even ones they know well and play against regularly, often for no other reason than the thrill of deceiving others into thinking that they are who they seem to be. I have observed quite a few players well known to me suddenly changing ISP's for a few days... this may well be them really playing from a different address or it may not be. Specific Ip address bans are not very useful in dealing with this issue as the griefers are knowledgeable enough to change them when they want to.
There is a nice little open source program called peer block, its primary use is to block defined addresses/ranges from having any access to your pc at all, there are a whole bunch pre defined lists of parasitic/unwelcome organisations, but you can also make your own lists. So if you get Griefer problems, do a Whois on their IP, get their ISP's address range and add it to your list.
I have a little blue button on my desktop that automatically prevents anyone using a particular ISP from being in my games, I call it my Lag Bomb, It affects 3 BZ2 players that I know, 2 of them are not unwelcome, but I don't see them very often (and could simply exempt them if they showed up), The other player is The Griefer and I can lag him out of my lobby/game no matter who he is disguised as or which Ip he is using that day. After clicking the little blue button, it usually does an update, which gives you just enough time to deliver an appropriate message via bz2, It really is quite a lot of fun

. It could also be a useful test tool.
I recommend peerblock to anyone who does not want clearly deranged Knob-ends in their games.

The Silence continues. The War Of Lies has no end.