I don't know of a way to disable the startup repair, especially as there are those times where you'd need to use it. In situations like this the real fix is to teach the users NOT to unplug the computers. I don't know why they are habitually trying to destroy the machines, but this is a human behavior problem, not a technology problem.
If you're an outside consultant (you refer to them as clients,) bill them for your time while telling them not to do this. Move where they're plugged in so it's inconvenient to unplug them. Anything to drive home the point to not do that.
Plus, the reason it needs to run startup repair is to make sure the filesystem is in a usable state! You'd want it to check this to prevent corruption. Making them sit through the repair cycle might also drive home the point: DON'T DO THAT.