I've got an old forum I would like to revive, it gets a decent amount of visitors, and about 5 new people registers every day, but everyone seems to make identical support threads, then leave (I offer free templates/scripts). I wiped the forum clean, and upgraded to IPB (from phpbb), what can I do to try an encourage people to come back? I'm not really interested in paid posting, as from previous experience, I tend to get "crap" posts spamming my forum, which attracts the wrong kind of people.

PPC is the way to go if you want to boost your traffic and promote your forum. The results in any Adwords campaign are very immediate. You could also try doing an online contest or something to attract visitors.

I agree with mrandrei, to add some more you can also try to put more interesting details on it to attract visitors and be able to convince them to join in your forum.

There are several things you can do to revive a community. There are four golden steps, in my opinion:

1. Realize your mistakes and the causes of failure.
2. Publish Content and Posts
3. Promote your Community
4. Upgrade your Forum and add new Features

Article: Reviving a forum community.