Remove/hide iTunes purchases from other users on my own computer.

i let some people borrow my computer so they fill their ipod with music. they bought music through itunes, with their account. their purchases show up in my library though, even though i'm signed in to my account in the itunes store. how do i hide these songs that i did not buy. i can listen to them as if i bought them, but i didn't.

i know i can just delete them from my library, but is there another way?

on a further note, why do i have access to these songs that i didn't even buy?! they're not from my account, so why do i have them at my disposal?!?!