I leave Mag-Floats in all three of my tanks, and haven't ever had anyone touch one. Like you guys, everyone wants to know what they are, and they're fascinated when I explain that it's a cleaner magnet (they're even more so by powerhead magnets), but it doesn't seem to prompt them to start messing around with my tanks. My husband won't go near anything fish related without asking me first, unless it's an absolute emergency (he's saved carpet surfers more than once when I wasn't home) - I guess that's just how we work. I don't touch his computers, he doesn't touch my tanks. We don't mess with our friends' stuff, they don't mess with ours.
Kids I can understand, especially little guys, because they're very curious and don't know better. I'm actually surprised though that adults do this too - I can't even imagine one of my friends behaving that way. I'd set some boundaries for those people who just can't seem to stop themselves - let them know the potential damage they could cause by trying to 'help'. It's very likely they have no idea they're scratching up the tank and damaging the seals, and I'm guessing once you tell them, they'll never touch your Mag-Float again.