That would be a great feature if you could tell gmail what +something suffixes you've used and it blocked all mail to other suffixes and the address without the suffix. As it is, spammers can just strip off the +something part, or change it to +somethingelse, to cover their tracks. I know this is gmail specific - for other providers they'd have to keep +something suffix in case it really goes to a different inbox - but gmail is so big that spammers can afford to make a special case for them.
Doesn't Gmail support rules that apply when no other did? That could be used to filter out all emails to addresses that are in use we while deleting all others.
