When a minor has that event and the race they stole from was powerful, I instantly change strategy to get the techs from that minor.
The simplest way, I've found, is to threaten them into gifting them to you. That is, get an invasion fleet overhead, declare war, then offer peace for that price.
One trick is to gift them a planet of yours deep in your space (after you clear it of all but one population, of course). Then, even if some other race invades them, they remain in the game. Meanwhile, you can invade their home planet, they still exist as a race, and then offer peace for the price leaving them with only the cheap planet you gave them.