Now the first thing that you need to do, is to make sure that your roommate is actually the one who is up to the shenanigans which have been causing your problems. This is because you need to have a reason to get back at your roommate. Should you not have any reason to get back at them, then it would be you who would prove to be the bad roommate and not them.
This is why the first step is to always check what is going on, before you go on and do something about it.
Once you have gone on to identify your roommate as the one who is causing you problems, you may start to plot your revenge. Now there are a number of ways for you to do this, but the best thing to do, is to line up a series of pranks, which would end up getting back at them, while making it look like you were only kidding.
This seems like the smarter thing to do, since this way they will learn their lesson, while failing to recognize that they are being toyed with.
The last thing that you can do, is to go on and cut off the basic things such as electricity and water on their half of the place. This will send your message across loud and clear and they will approach you to check just what is wrong, when this happens you can explain to them just what the problem is, without actually getting into trouble.