Deleting objects boggling me

Hi. New to Alice and the site so please forgive my pedestrian question. It's driving me batty.

I'm doing a city skyline with various planes, helicopters, ostriches flying over it. Completely simple, beginner stuff. Problem is, I can't figure out how to delete the objects when I'm done with them. So they just sit offstage (outside of lens angle) somewhere. Sometimes I use them again and when I do, I want them to come in at different points then where they are 'waiting'. So then I have to figure out were they ended up, 'fly' them around the lens angle, line them up and have them come onstage at a different place.

Is there a way to 'delete' an object after use? I'm sure this q has been asked and answered but I couldn't find it with searching.

Thanks in advance,
Blake