I added an "other" vote, along with Remove their AIP, Make them higher-mark, Allow them to aid defence, Have them attack/defend if AIHW attack, Let Starships/guardians join them, Form starship/escort fleets, and Prefer low-cap ships.
My "other" option is to stop them ever becoming Threat or pathing onto human worlds while patrolling. The way I would like to see Special Forces is as small but powerful roaming wolf-packs of Starships, Guardians and fleet-ship escorts, maybe one mark higher than the wave level, patrolling between SF guard posts. They'll avoid human-controlled systems altogether (I don't really care if they avoid neutral systems or not, but they should probably avoid the Dyson Sphere system). If they path into an AI system that's under attack, they'll join in the defense. If a high-value AI system (or one containing a Special Forces Alarm Post) near a wandering SF patrol comes under attack, the Special Forces will blob up and rally to the defense. If they succeed in killing all the human attackers or chase them off, they could just go straight back to patrolling, or hang around the attacked system for a while in case it gets attacked again, or maybe they could try for a counterattack (the counterattack logic would need to use something like the Threat firepower calculation to make sure you can't easily bait Special Forces into charging into a meat-grinder).
The exact makeup of Special Forces packs - higher-than-average mark numbers, low-cap ships, Starships, and possibly Guardians - is primarily an aesthetic thing, although it's also to make them feel different from the normal fleet-ship defenders. The main focus would be on converting them from something that randomly attacks you (and gets ignored, because it's like a slightly pathetic version of Threat) to something that reacts to what you're doing and might foil one of your attacks every once in a while.
Oh, since Special Forces would never path onto human worlds, there are a few side-effects:
1) Special Forces guard posts ON human worlds should self-destruct, because they're pointless. Because of this, the AIP for SF guard posts should be removed because there's no real way to avoid killing them.
2) If you manage to isolate a Special Forces guard post such that the wolfpacks can't path to another SF post without going through a human world, they'll stop patrolling and just camp the post.
3) It's possible for Special Forces to get completely stuck and unable to path to any SF post at all, possibly because you just killed the last such post. In this case, it's probably OK to release them as Threat.
4) Special Forces Captain goes from from an aggressive to a defensive AI type.
5) Special Forces plus AI Eye is an absolute pain, since the Eye forces you to use small attack fleets while Special Forces turn up and randomly squash small attack fleets.