Don't know the actual numbers, but since no one else answered I did a bit of testing.
However much executing them decreases encounter rate by, it's not a lot. I noticed that after doing it I'll usually get Fortuna and the Minotaurs more than once each before seeing another Blackguard, but I still see them fairly often.
It is repeatable, but doing it repeatedly does not further decrease the encounter rate that I can tell.