When he "stole work for you", do you mean he stole contracts from you and scratched your name out? Or, do you mean he did better work than you, was easier to deal with from a customer stand point, or advertised a cheaper price? What I'm getting at is your upset your loosing work to someone doing a better job that probably wouldn't matter anyway if he had the proper license.
If this isn't true you have nothing to worry about because most rational customers will realize you were the better service provider and switch back after a year. Why blow it with your old clients because they heard a rumor that you turned in a lawn-service for operating illegally? Honestly, I bet most my customers think I run some tax-evasion operation.