Throw your trash all over the place. One thing that annoys your room-mates is the fact that he/she has an unclean room. You try throwing your trash all over the room and that will bug them as they will have to clean it up at one point in time. It is definitely something that will annoy them as it will also make them work later on.
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Try playing your music as loud as you can. In fact, try singing along the songs that you are playing. It will be even better if you know that songs that annoy your room-mate and if you keep playing them at full volume, and sing along to those songs at the top of your voice, it will really annoy them to the limit.
Try throwing your dirty clothes in their area of the room. Once you are done with wearing your clothes, just throw them off wherever you feel like. On their bed would annoy them the most as that is the area that people really like to be clean as they have to sleep there at night.
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If they have some special food left in their fridge, take it and eat it without permission. Try not to order your own food for a long time as they will be able to get back to you by eating your food. Hence, try and eat their food whenever you get the chance.
Disturb their sleep
Whenever they are sleeping, just start screaming at a random TV show. Switch the television on and turn the volume as loud as you can. Even try screaming at people outside your room.
Have your friends over
With your friends over in your room at all times, you will have the chance to annoy your room-mate as much as you can. You will also have the strength of other people in case your room-mate will try and pick up a fight.
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Tell them they snore
When they are sleeping at night, randomly wake up and start screaming at them, saying that they snore like a train. Keep screaming at them for a few days and it will really annoy them.