as far as I know you wouldn't or can't delete users, contacts, products, etc... I think this is due to potentially having many other things in a system that depend on such 'objects' being there. The common way to get them out of your way would be to deactivate these. Like this all dependencies are still met, but they disappear from the durface. There is field which says 'active' which has to be unmarked in order to do so.

I wouldn't test that with anything you still need though because I think might be hard to find such inactive objects back again. I guess you might even have to crawl directly into the database to change the 'active' field of such an object from 'FALSE' back to 'TRUE' again.