I've thought of trying that for the forums I run.
Though, on the other hand, legit people will pretty much always post right away too. So it might divert new members, but maybe not. Some spammers actually accumulate tons of accounts then suddenly strike with all of them at once. So maybe deleting accounts that have been innactive for more then 24 hours could help too. Really it's hard to decide as they vary their ways so much.
Actually, just block words like cialis, ipod etc. If someone is posting and these words show up, it racks up "points" (like an email spam filter) and then auto bans them. Only run this scan for the first couple posts.