A friend of mine wants to add support for PayPal subscriptions in their phpBB forums. I did a little digging and found two options.
This one allows you to require that users subscribe via PayPal before they can use the forum at all.
This one allows you to specify a group within phpBB that requires subscription.
I haven’t tried either one yet, but if I had to pick one, I would try the second one first. Why? Because then you could offer both free and paid forums. The free ones could be the teaser. The paid ones could be limited to a specific phpBB group that you have to pay to join.