Okay, so your first problem is that if you set up a wifi camera and your roommate has a phone or laptop, they will probably be able to see there is a wifi camera connected to your network. Second, how about getting a new roommate?!?! I wouldn't stay with someone I couldn't trust to not burn down the place, what else can't he be trusted to take care of? If you insist on keeping him as a roommate, but honestly can't trust him with the oven, why not talk to him about it? Or just keep the fuses for it in your backpack or something whenever you aren't using it, that way he can only use the microwave when you aren't around.