usually at some point the dorms get a fatty delivery of the phonebooks that are going to go to each room in the dorm building
so what my friends and i did was individually tear out each page of said phonebooks and toss into friends (the pranked) room
yes it was very timeconsuming but also quite fun to witness
a dorm nearly full to the brim with crumpled up phone book pages...
you can plastic wrap some ones room as well
and if you do it well enough they will never see it until they start trying to use all of their belongings (laptops are my favorite)