Try using Security: Barrier Balm. Also have your clients use Hostage, which is a DHA friendly hand sanitizer, after their spray tans. This will only help when they are awake. You may want to do the spray tans earlier in the day, so it has time to activate before they go to bed for the evening. You mentioned clients, so this is happening to more than one person?
Sometimes the best thing to do is go over what should and shouldn't be done after the tan. It may be that you just have to go over it again. You might also want to walk your clients through what they do while the tan is developing. They might not be realizing what they are doing.