Write a letter to all your customers asking them to keep their eyes out for this guy and what he is doing?
Regarding the customers he has taken from you, go back to them and explain what has happened and ask them if they will continue with your services. If they say no, don't lose any sleep over them, they are obviously price orientated and therefore would have dropped you when the next w/c came by with a cheaper price?
As for the individual himself, two options really.
Option 1 - Hit him, face assault charges, possibly spark a very dangerous game and who knows where that will end up? Let your customers see how your reasoning methods etc........
Option 2 - Hit him where it hurts, i.e. be one step ahead, get your customers on board with you, tell any new customers you will never just give up and would always notify them first, make a joke about this guy and explain if he turns up don't panic! Report him to Inland Revenue but most importantly don't let him effect your business by losing your head!
Box clever!!!!