If you want to prune members that are answering to a certain criteria just go to your acp --> member tab --> use the advanced search function. Fill in what ever you want as criteria and then when search is complete prune all of them.
Or there is also an hook/task for managing inactive members: Manage Inactive Members 1.2.1

And for pruning old topics/posts just use the(M31) Delete Old Topics 1.0.0.

I hope this helps you

Ps: if you can not download the hooks if you do not have an active license, just pm and I will download them for you