I just delete them one by one. However, you could gather them together in one house by going to manage worlds, and then, delete the household.
You could make more Sims and put them in houses so they don't get culled (deleted), and then, fewer game generated sims seem to appear in manage worlds.
If you want to keep them, but change their appearance, personality or name, etc., then you could open the cheat console (box) by pressing control + shift + c at the same time. After you're in the box, then type testingcheats on.
Then, type cas.fulleditmode
Finally press escape to get out of the box, left click the sim, and click on modify in cas (create-a-sim). You can change anything about a Sim in cas.