if you just kick them out of the active houshold's home then odds are you will see them again. They'll land on their feet some how and find a new place to live.
If you don't want to see them again instead of kicking them out go into edit town mode and click on the active household's lot. Then in the window that pops up with all the info on the selected household click the "Split Up Household" button which will look like a few people separated by a vertical line.
Then you'll see a window with two households, the active household on the left and the household to be moved out on the right.
Move the sims you want to get rid of to the household to be moved out. Then they will be moved out to the clipboard where you can place them on some other lot or if you prefer, just click on the clipboard and click delete clipboard and then they'll be gone for good.
