The way I would see it working would be to present exactly the same slider as with the Store Area function, but in this case the slider would represent the zooms levels of the contained area to be preserved. All zoom levels beyond the slider should then be erased from the map cache, in such a way that those tiles would be subject to a fresh download the next time that they are displayed, in on-line mode. Hopefully, by re-using a lot of existing code, this could be achieved without too much extra programming effort

Furthermore, should this function detect any corruption in the cache database, it should attempt to repair it, or map the affected pages out of the database if it is beyond repair.
IF physical space can be reclaimed from this process, then that would be a nice to have, however, if this is not practical then perhaps an external database re-organisation (compact / repair) function could be provided at a later date.
Would you please add this to the requested features list.