vBulletin (the software that runs this board) comes with the feature that you want already.
Check their documentation, here. I am not sure if that feature is in the 2.x series, which is what Allen runs (see the bottom of this page), but it is definitely present in 3.x.
Deleting users after inactivity is generally not good practice but
disabling accounts is. What happens when a troll that knows the inner workings of the system waits for a well-known account to expire, and simply registers with the newly expired name? What if an old veteran returns to report the long-term success of the DJ system, after a two or three year absence?
Ideally, "account disabling" should be a passive feature, eg: "you haven't logged in for six months; you must respond to this e-mail verifying that you still want the account, or else it is disabled." Users can re-enable using their registered e-mail ("send me a new scrambled password"). But as far as I know no such features are in vBulletin.
Anyway, it's probably just better to filter the members list so that it only shows non-banned members. You ban accounts over time with the prune feature.
Just programmer talk; don't mind me...