I just asked this question in another thread.
The general consensus was that if you want to keep some around, it was possible to do so as well. You can leave them a group of 10-50 systems, beyond which they don't typically expand. Or at least expand rapidly. And you can additionally border that area by raising the T score of surrounding planets, which reduces their habitability.
This is my plan for my Acer II system. I don't want to kill them off completely, because they are a key part of the original game, but I do want to keep them contained, and lower their nuisance value.