I just don’t wasn’t then to waste their time locally blocking a clear spam account, that should (and usually will be) removed by the originating instance in a couple hours.

I am afraid, that removal of spam account does not necessarily federate to another instance. On Mastodon activities federate mainly to instances an account, which made/edited/deleted the post, is followed from. (Or an instance can subscribe to relay, which exchanges all activities between all participating instances).

Report always landing to your admin may be something good in light of this…