It's a bug with the baktrian hippies model. If you know your gonna win, use the "auto_win" cheat to win, or just autoresolve normally if you don't. Then go into the export descr_building file. Delete every line with "hellenistic cavalry hippeis baktrioi" in it. Then baktria won't be able to recruit them, so it's only a matter of killing the ones they have left, and then no more of this bug.