Quoted off another list:

I often "fire" troublesome customers , in the nicest way of course
I refer them to my competition with some excuse like one of my machines has broken down or I don't have stock etc...let them tie up my competition...
If they really persist , we then start a FOAD pricing structure (F... O.. And Die) in that the price is so prohibitive it scares them away or if they do accept the price , there is so much jam and icing on the cake that one can put up with any aggro you gonna have.