Kevin Twohy has a list of heuristics for new projects/clients, and my favorite is simple:

No Bozos. Simple policy. No exceptions. You know it when you see it.

Every founder has a story about the time that they ignored the red flags and bent over backwards for a particularly standoffish or problematic customer/prospect/client/hire. It's easy to get lost in the details of the anecdata (was it a bad policy to even entertain the request? was the user technically correct, just a jerk about it?) and therefore slightly more hard to form specific policies to prevent that exact genre of scenario.

But it's very easy to make a policy of "no bozos". Everyone innately knows what it means.