Perhaps a couple of your customers, especially those with whom you were successful despite the badmouthing they heard, would act as references...maybe just a note about how your company performed excellently, which was a pleasant surprise after another local company badmouthed you and made them think the whole industry was a bunch of stinkers.

This kind of note should not name names; it would bring up the issue that those who badmouth make everybody, themselves included, look bad; then it would reassure your potential clients that this is not so, at least NOT WITH YOU!