they make screws that turn in but not out. you should be able to get them at any hardwear store. they also make screws with little holes in them that require a unique tool to get in and out. but not sure where to get those.
Auto Zone and Pep Boys both sell "security screws" for licence plates. They are basicly an allen head with a post in the middle of the hole, so you need the included allen wrench with a hole in the middle to install/remove them.
Of course anyone else who bought the same screws at pep boys has to tool to steel your plates if they want.
I bought a pair of nice looking chrome licence plate frames, and they looked great... for half a year. They didn't last one winter here in Connecticut, they are all pitted now. If you are spending any ammount of money on them, make sure they are good quality otherwise, get cheep ones and count on replacing them.
I am on my 2nd set of frames, the first set lasted just as poorly. I am thinking of sanding the chrome off the current ones, prime and painting them either silver to match the body color of my car, or maybe black, I'm not sure yet...
Of course if you live in someone place that's warm and sunny all year round you might not have the same winter problems I do.
Just make sure the license plate frame doesn't cover any vital information on the license plate itself (such as the state name or plate #). When I got pulled over for my tint (2nd time) I not only recieved a ticket for my "illegal tint" but also for my Rowan University license plate frame since it was partially covering the words "New Jersey." Apparently he was jealous that I have a 4 year degree and am making about 2X as much as him. Well, everything is off now and I get to goto court on May 6th to see if I can get these fines lowered/dropped. If there's one thing that pi$$es me off it's pigs on power trips... Peace.