As this is a problem that you are facing with quite a few of your regulars i would be inclined to put up a notice in the salon stating that as a solarium owner you have certain rules and regulations to follow, set out by the sunbed association,
and would all clients please note that these rules are there for their benefit and protection,
therefore you are only able to allow them their recommended time on the beds,
I'm sure if you get in touch with the sunbed association they may have a poster with the guide lines that have to be followed that would reinforce your notice to the clients, hth :hug: