What I do to remedy this is to have a row of solid chunks at the top of the layout, making the camera boundary 1 chunk below the top (pretty sure it's $100 if 256x256 chunks, $80 if 128x128 chunks). What they look like is up to you, they just have to have a solid bottom row of blocks. This way, players can't head over the top boundary no matter what they try. It may reduce the size of the playing field a bit, but I'm pretty sure that can be compensated for. This is, of course, not exactly a perfect fix, as the amount of usable level space is decreased slightly, but alas, I can't be bothered to come up with a true fix at the moment.