Are the ip's in a certain range or a particular country? You can block groups of ip's or ip's for a whole country with .htaccess - it's a bit drastic but might stop a few. Otherwise I've not seen a foolproof method other than manually checking and deleting offending accounts.
00. limit posting/replying to registered members
01. ban his user account
02. add image verification upon signup
03. add email verification upon signup
this will atleast reduce the number of unwanted registrations
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