As there's recently been a discussion on what's appropriate to tip your pizza delivery guy, it got me to thinking about how people handle tipping when the service is genuinely bad.
I've worked in restaurants (both on the line, in management and also as a server/bartender) so I tend to be a bit sensitive towards my hard-working brethren. There have only been a few times where I felt forced to live a small tip due to genuninely bad service, but I always made sure the server knew why or involved a manager.
So, do you have a policy for dealing with this? Have you ever left nothing as a tip?