Just don't interact with them. For example, if you visit their secret planet, you will suddenly see them in your game. Or if you talk with them in your world. So don't. Also. as someone mentioned, delete them in the "household" section of CAS. Eventually, they will disappear. As for abductions, that may happen, but it was extremely rare in my game and seemed to occur only when I interacted with some. It's actually funny when it does happen and if you aim your camera right, you can watch for when they bring your Sim back in front of your view, lol. The last time it happened in my game was a long time ago, though.