Two thoughts here - 1) do not allow users to select a password, instead email it to them. If it then bounces back, then it should be automatically deleted, either that or every so often purge all those with zero posts.
2 ) If you are getting this problem mario2, can't you remove the catch-all from your domain, so that only the emails addresses you choose will be valid. This will cause all these others to bounce.
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at the moment - but expanding all the time....) 
