[zaarn]

How would anyone go about preventing you from doing that?

Keep in mind /dev/mem is just a convention, I can just mknod it at any place in the filesystem I want. I can even put a FUSE overlay on it that would turn it into a regular file instead of a device node, so you wouldn't have any possible way of telling what you're about to do.

There is no "defense" against /dev/mem, that's just something you don't do as a sane programmer.