Why not have a mass prune of members that have not been active for over a month?

This should get rid of a few non-active members, then each month you do the same. I dont know how much work this involves but a forum that i use has done this for a while but they use 2 weeks.

They also have a subscription method where you pay around $12 for a year, this stops you from being pruned and allows you access to a few other things, maybe you could use it for prem links to a series of your choice or something.