If you can't make the sim leave by clicking on him/her and choosing "Ask to leave" then you may wish to try sending your sim or sims to a different area of the map, have them visit a friends house or the park or something and hopefully when you go home the imposter will be gone.
If all else fails as a last resort you could try moving house as that tends to fix a fair few problems or try enabling the cheats "At the start menu, hold down the LB,RB,LT and RT at the same time to enable cheat option" and resetting the sim, though that will most likely disable achievements on that save file.