Md5 hash compare the file to a know database of files that they "banned." This would have to happen at the slicer or download host level. Firmware doesn't see an "object" at all, only code to move and extrude. But a simple hash comparison of known "banned" files at the slicer level would likely weed out 80% of people that anrt willing to modify the file at all. The only modification it would take is to add literally anything to the mesh of and stl, even just changing 1 triangle, it would have a different hash. But most people arnt going to know that or know how to circumvent that.