Other than purchasing equipment to do prints at home are there any precautions that can be taken to prevent a print lab from maintaining and possibly even giving out or printing additional copies of a piece? For example some print shops may print a piece to hang on their walls of stuff they've done - is there any real way to prevent this?
There's no value in any print provider using client files unethically. Word-of-mouth spreads.. soon they'll have a hard time finding any return clients if not any client.
Print providers have a right to a portfolio the same as the designer has a right to a portfolio. It is well within reason that a print provider maintains copies of past work to show off their capabilities to potential clients.
If you fear your print provider is unethically profiting directly from unauthorized duplication, well then you have a legal issue to follow up on.