There's already an "Activity penalty" if enough posts get deleted. The next step is a ban. I don't think most spammers care about Merit or Activity anyway. And not all deleted posts are bad, such as scam warnings while waiting for a post to be deleted.
Correct, though I'm pointing toward reported offenses that have been deleted. For enough posts to be deleted where someone
does get an activity penalty, it would have to include all of their posts within that activity period. An easy workaround is to make a simple post in an off-topic board with a barely-passing reply - it shouldn't be deleted, and reserves activity.
There are still dozens of users out there that will consecutively post absolute rubbish one-liners. You don't even have to look for long to have enough reports to take up an hour's worth of time submitting. Bitcointalk is an orchard of low-hanging fruit.