It depends on how long they are there for. If it's only a couple of days, just turn it off and say nothing. If they are there long term, perhaps you can state in a nonchalant manner, while they are nearby, that someone keeps leaving the lights on, and turn it off to make your point.
yer, ask them if they could turn them off, leave a fake and really expensive power bill on the bench where theyll find it or if theyre there for a while (over a week or two) you could get those timers that automatically turn the appliance theyre connected to off.
ask the guest to nicely stop... or kindly ask he/she to leave... tell them that there are rules that need to be followed or even show them good manners... but the best way to deal ths situation is to confront them
Kindly remind them that you'd appreciate if they remember to turn it off.
remind them that they're not paying your electricity bills!!!