Welcome to Windchill. No selective delete. No purge selective revisions. No delete everything but the latest X versions. You have two options. You can delete everything from the latest backwards using the "Delete" function or you can use Purge from the first iteration forward (but never get rid of the last iteration of each revision).
As long as A.1 is the first revision in REV A (no A.0), you should be able to use purge for this. I'd recommend temporarily moving object to a separate, empty product (or library) while doing this. It makes things much simpler.
Tom U.