Maybe. We could add a requirement for a minimum number of total votes as well. But that's why I think we could just make the threshold be "very low": A post with a total rep of -20 is clearly "excessively bad" in the view of a critical mass of voters. And remember that this is only for hiding the post; it does not delete it; so we don't have to worry too much about being too aggressive.
Another point which should be made explicitly, is that this "hidden"ness is dynamically determined. A post could float in and out of that state as the voting goes on. Also, the threshold parameter can easily be tweaked to try to make it more optimal.
I reckon we are the only monastery ever to have a dungeon stuffed with 16,