I used to run a catchall and the problem wasn't addresses like admin@; it was popular names. Turns out some spammers just take a list of common email local parts and email those names at every domain they can find.

There's a silver lining, though. When I took down my catchall I took the 100 most popular guessed addresses and routed them all to my Bayesian trainer. It was a big help with certain sorts of spam.