Pretty easy, and superficially appealing, but I find that there must surely be exceptions to a rule such as this. Nuclear weapons (and similarly, some chemical weapons) are the obvious special case.
Edit: reminded reading a comment below that child pr0n is another one that the general moral consensus has problems with (I refer to the 'consensus' not because I think it is always correct but simply as it provides cases worth thinking about), and also handling stolen goods (although this can be justified through property rights).
Seems kind of like the "Godwin's law" of rule of law. The fact of the matter is that we simply don't encounter these sorts of situations often enough for myself to understand using them as policy setters.
Seems to me that in that case, banning owning this hypothetical something would be redundant. Just use the laws that already ban doing whatever it is that they are doing.
Of course, your milage may vary. I also support repealing intoxicated driving laws, since I reason that reckless driving laws already cover that sort of behavior. For reasons that I don't really understand myself, people tend to think I'm off my rocker there too ;)
Edit: reminded reading a comment below that child pr0n is another one that the general moral consensus has problems with (I refer to the 'consensus' not because I think it is always correct but simply as it provides cases worth thinking about), and also handling stolen goods (although this can be justified through property rights).