I you plan to mark them in a semi permanent fashion, then that would probably best be modeled as inflicting consequences on them. Did you plan to do this after/ as you were defeating them? Or something closer to mid-way through the fight?
If you planned to do it as part of a defeat, then you can probably just narrate them getting "marked by you" as a (long term) consequence.
If you plan to "mark" the baddies as something that happens mid-way through the fight, you could try something like getting the lethal weapon stunt and the killer blow stunt. Then, with a fate point, your fists would be weapon: 5 for a single strike. Instant consequence land for mortals.
Yet another option, if you are going for a more cinematic feel (any you want to be able to use it against monsters that heal really fast as well) is to try convincing your GM to let you have a rules hack stunt mechanically similar to addictive saliva. Where, instead of licking people and putting the "addicted" aspect on them, you punch them and put the "marked by you" aspect on them,