spammers - how can we catch them?

[Dragon]

(yes - I searched)
they log in and out i Lobby too fast to catch them
so, for now , we

[FONT=serif]pwd[/FONT]

protect certain channels
do not want to

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protect the whole server
as we invite visitors to use it (BF2)
how about some cache for users IP when they join
the data IS there when we ban them
just keep their IP in a log so we can fetch it to add to ban list

[FONT=serif]manually[/FONT]

after
they leave

or

[FONT=serif]is[/FONT]

there another solution?