My first question would be why such a civilization isn't just colonizing that galaxy. If the reason is that those distances are too hard to cross, then my question is why they're concerned about other civilizations developing. Of the top of my head, the best reason they would be worried about it is because it is something they would do themselves; but ever for an advanced civilization that seems like an expensive project with limited return. Or maybe they're as fearful as Larry Niven's puppeteers.

It also would be more efficient to go to that galaxy, tow its stars around, and form new black holes there. You'd move much less mass a much shorter distance. You could probably even pick a relative handful of stars and just nudge them and wait; they're already moving after all. Might as well use that momentum.