I've never had service that I deemed to be "untippable." If the service were bad enough that I wouldn't leave a tip, I'd probably just talk to the manager. Be doing the restaurant a favor.
Furthermore, if you just don't leave a tip at all, most waiters would presume that you were just one of those a**holes who never tip "on principle", instead of concluding that you were trying to "make a statement" about the service. It's better to leave a substandard tip for substandard service, than to leave none at all.
Also, in my experience it's amazing how many people who make a point of loudly proclaiming "I don't think you should tip if the service is bad" manage to find reason to decide that the service is "bad" more times than not... e.g. "Hey, my water glass ran dry for thirty seconds before the waiter came by to refill it -- bad service, no tip for you!" Okay, that's an extreme example, but I've seen too many tightwads "justify" their crappy tipping on the thinnest of excuses, or because (from similar motivation) they set their standards of "good service" ridiculously high.
Hint: If you leave substantially less than the standard 15-20% tip more often than 1/5 of your restaurant visits, you're a cheapskate -- service isn't *that* bad *that* often. Or if it is truly that bad, it's because a) you specialize in picking trashy dives to eat in, or b) you keep doing something to inspire most waiters to snub you. And either way, *you're* the problem, don't take it out on the wait staff.