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I guess that since they think they are your new best friends, they won't consider themselves trespassers. Maybe the next time you go away, you can ask them to keep an eye on the tub since you just poured all sorts of nasty chemicals in to keep it from getting contaminated while you are away and you hope no one tries to use it..Bill, you are so tech saavy, you could hook up a camera and see what is going on. I know anyone can probably unlock the tub, but I still lock mine..I think if they tried to jimmy it open, you would see the marks on the clips..they are only plastic.
If you're only gone for the day or evening, killing power at the main breaker would dampen the fun a little bit. If you'll be gone for several days, chlorinate the hell out of it before you leave. (Gotta prevent nasty bugs while you're gone )
Anyone ever had problems with neighbors sneaking dips in your hottub? We have relatively new neighbors, the overly friendly type. A couple of times I thought it was my approaching senility. I am usually lazy and only secure the cover latch closest to the back door. One days when foul weather is approaching I may secure all 4 latches, but never only the one away from the door. Two weeks ago I changed the water, and last weekend we were out of town for the weekend and this week we have foam in our tub with the jets on. I haven't had foam in the tub in 5 years. (Suits rarely in tub) I guess I'm going to have to try and find those keys to the cover locks. I think I might put up signs about tresspassers will be shot.
You could turn the temperature down...save energy and discourage unwanted guests.