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.