I feel this should be a core feature of phpBB. Forum pruning already exists for removing this but not locking and/or moving them.
There are two clear paths to accomplish this goal.
#1. "Action Drop-Down Box" - Simply allow the user to select which action will be taken when an item qualifies for pruning (Delete/Lock/Move). This is would be the easiest option to implement.
#2. Individually set the days required for each action. 0 days would obviously mean that action would never be taken.
Auto-Prune Delete Age: XX Days
Auto-Prune Lock Age: XX Days
Auto-Prune Move Age: XX Days
I'm not sure this degree of control is required, however, administrators may want to move topics into a 'stale' forum on their board, leave the topics sit there for some more time, then finally lock/remove them if they still remain inactive.
Personally I could see a few uses for this, especially the automatic move+lock combination. For example, you run a contest every 28 days. Users post many topics in this forum during that timespan. The forum which holds topics from these contests can be setup to automatically move old-contest topics away after the 28 day period keeping it clean and clutter free for the next contest. This is just one example and I'm sure there are more.
I feel this should be a part of the core since it essentially extends the current Auto-Prune and has many, many, use-cases.