Generally you do this as part of the import – screening out topics and users you don’t want to arrive – not after the fact.
Most forums are chock full of shell / spam / bogus accounts that should be trimmed before they come in. Which is not too hard since they generally have no posts, either.
Agree, for this particular use case. In general, I think batch operations on users would be useful.
Use case: sort users descending by number of posts created, select those above a threshold, and bump their trust level - part of onboarding after a migration.