I worked for a few years as manager of an RV parts store and got to deal with some irrational customers. Early on I really tried to accomodate them but near the end, I simply sent them packing, telling them the names of my competitors. I figured that my business would improve and theirs would suffer from it.

I had one guy that bought some plastic plumbing fittings that are commonly used in RV's because they're immune from freezing damage. These fittings are designed to be installed hand-tight and I made sure I stressed that to this customer I sold a bunch of them to. Well about an hour after selling them to him, he returned with these fittings all mangled from the pipe wrenches he used to torque them down. I told him I could not refund his money and he went into this major rage, threatening to come over the counter after me.

This was at a time I was playing open-level racquetball and figured I could've hit him 20 times before he even saw the first one so I basically told him to "Bring it on". He took one look at this wiry and very confident person on the other side and decided that was not a good idea. I told him to get out of the store before I called the cops on him, to never again return, and to make sure to tell all his friends (I knew he had none) that our store didn't do business with morons and he stormed out.

That was a very satisfying day.