At a federal level, it's not that easy. If there was an easy way to ban them, the ATF would have already. It's really hard to define a bump stock in a way that wouldn't either ban a lot of other things, short of classifying them as machine guns. If you classify them as machine guns, at this point, you pretty much have to let people who already have them enroll them on the NFA registry (federally, there is no legal basis to confiscate property that was legally acquired and legal to acquire when acquired), which is not necessarily what you want either.
At a federal level, it's not that easy. If there was an easy way to ban them, the ATF would have already. It's really hard to define a bump stock in a way that wouldn't either ban a lot of other things, short of classifying them as machine guns. If you classify them as machine guns, at this point, you pretty much have to let people who already have them enroll them on the NFA registry (federally, there is no legal basis to confiscate property that was legally acquired and legal to acquire when acquired), which is not necessarily what you want either.