I had the same problem and put it in a letter to all the team. I clearly stated that nobody is to leave a clients address early unless it is a family emergency or you have contacted me personally for permission. I also said that clients had raised concerns and this could result in us losing clients and in turn people losing hours.
I know it might sound a bit draconian BUT at the end of the day the client is paying my and their wages and their continued custom is more important than a team member leaving early cause they want to get home to watch TV. I do have a relaxed approach to the team but their is a line which is not to be crossed. My husband does the admin side of the business and keep out of the day to day dealings with the team. So when an issue has to be addressed , He does it so they know it's important and they can't use their friendship to not follow the company rules.
You take all the responsibility for your team actions and payout on wages, uniforms, insurance, advertising etc and put a lot of work and money into the business to build it up and keep it ticking over that the employees never see. So you either draw that line for them or they will take advantage. You can replace a team member but it is hard work finding clients.