Sorry, they can't. Trust me, I've tried everything! But if you want to be rid of a Sim that badly, here's how to do it: When a Sim is selected (when they have the diamond above their head) click on them. It'll give you two options: Have Birthday and Ultimate Makeover. At the top, there should be your SIm's name and their portrait. Click the small red circle with a line through it, and you can remove your Sim from Simtown. Hope this helps!