Well is learned in my Ironman match that if you cap the Unbiddens growth right away there aren't much of a threat. Of course I proceeded to suicide the portal and emergency FTL out with what ships I had left.
In the ensuing wars when it was done. I got tired of starting new wars for conquering people an I just sat fleets over their planets and bombed them until they were gone. The planet I didn't want at least. On full bombing it takes a few years to clean up an empire.