"One trick is, if you know of a Ruins planet in an area you cannot hold onto for long -- send a small (1M population) colony mission there. colonise the world, "pop" the ruins ... and then abandon the planet. You get the Ruins' tech, noone else does, and you leave no target behind to be shot at."
Wouldn't it be more entertaining to build a bunch of fighters or mines or a WP?
Roadblock..at least for a little while.
Or just let them glass it; no huge deal. It's only if the planet gets captured that the problems can pop up (your population can be quite useful to other races)
Phoenix-D