If someone is registering, they sold be willing to make at least one post to keep the registration. Why not edit the welcome message or success message to say that if there isn't at least one post within a day the registration will be deleted?
There are a lot of members who login to read, but have never posted.
Forcing them to post something isn't my first choice.
And besides, once spammers figure it out, they'll post something innocuous to start with, leave it a while, then post spam.
Ideally I'd like all new registrations to be on pre-moderation automatically, but I can't figure out how to do that. That way first posts would always only be seen by me until I okayed them. Might frustrate a few people who post just after I go to bed, and have to wait before they appear, but it wold keep the spam out.