An anti competetion agreement helps in this instance, but you can always fall back on the business and profesions code which would open the bandits up to some serious civil litigation. Was there any formal WRITTEN agreement before? Find out if they have business licenses, liability insurance, workmans compensation insurance etc and go talk to an attorney. Then notify the local AHJ's if there is no licenses and insurances for these guys, that will put a damper on thier business.