Just to follow up for the benefit of others who might have the same
question. I had a look as per Eduardos advice, and I found the following:
External Categorizer Commands =
Which allows you to call a command to a program that filters for you. So
instead of having one rule per user I want to avoid, I can just write a
script, and add all the usernames I want to avoid to that killfile, and if
I understand things correctly, the rule with an External Categorizer
Command will run my script and return if there is a match, and then I just
select "delete message" or something similar.
Best regards,
Daniel