hg rollback reverts the last transaction, so you’d be left with unfinished merge, which you have to use hg update -C to get out.
If you don’t want *b (you have it in another clone), then enable the built-in MQ extension and run hg strip -r <*b>. It will get rid of *b and *merge. By default it saves a backup in case you change your mind again.
UPDATE (per @Rudi’s comment: sorry I missed the “already pushed” part)
Since the merge is already pushed out, NEVER EVER do what I suggested earlier. Hate emails from fellow developers would have been the best outcome.
Do this instead:
hg up -r<*merge>
hg revert -r<*a> -a
hg ci -m "undo unintended merge"
Or you could be more kosher:
hg up -r<*merge>
hg backout -r<*merge> --parent<*a>