If the screw is recessed and spinning in the plastic, a very long thin set of needle nose can sometimes grab hold and pull it out. They also make a screwdriver that can hold the head of the screw on the end of the driver. If there is room for this tool you shoud be able to use needle nose though. Lastly, you may be able to apply pressure (pull the pieces apart) while turning the screw and get it out. But if the screw head is stripped then Jerry and Greg are spot on.
Like Greg said.. Heat.. We heat up a Phillis driver or flat blade with a heavy duty soldering gun along the side of it and let the heat transfer thru the screwdriver to the screw head and then after a little heat back and forth until it starts to move. Watch the heat so and it dose take time but can be done.. I found on a few LGB Eng. they had some Lock tight on a few places and it dose need heat. This is the last resort if not able to get them out with a needle nose like Jerry said..