User Pruning - fudforum/FUDforum GitHub Wiki
The User Pruning admin control panel allows the administrator to remove users with zero posts that haven't logged on after a certain date.
Once you click on the prune button the forum will calculate the number of users that will be affected by the pruning and ask you to confirm that you indeed wish to prune that number of users. If you confirm the process the affected users will be deleted.
It is a good idea to run the Compact Messages utility after doing user pruning.
User that haven't logged in after this number of "units" as specified by the 'Time Period' option, will be pruned.
Whether the indicated 'age' is the number of days, weeks, months or years.