> why don't the owners of these properties release fart spray everywhere and turn the heat up to max until it is unlivable?

If the okupas have been there for longer than 48 hours without you trying to get rid of them, doing something like that could be considered harassment and/or endangering the occupants' health, so you'd probably face criminal charges for it. You're not allowed to alter the living conditions to force them out, but you need to (within 48 hours of the occupation) legally get them out of the property.