Have you guys have raid instances "stolen" lately? What, if anything, have GMs done to help you? How did you avoid it in the future?

Timeline (on Smolderthorn server):

1. Rebirth kills Twin Emps and pre-C'Thun trash
2. Later that week, a Rebirth member hosts a pug MC. A member of Decadence is invited and zones into AQ40, getting the instance ID.
3. Later, Rebirth runs Ouro. Two members of Decadence zone in to the AQ40 and attempt to wipe the raid. Some Rebirth members cannot enter the instance because there are some Decadence people in there. Rebirth wipes under pressure, calls it for the night.
4. GM tickets are filed, GMs do nothing.
5. Decadence zones in and kills C'Thun and Ouro on a later day.

What recourse does Rebirth have, if any?

This sounds like something for your own realm forums. The way you've written it especially sounds like you're just raising AD drama.