ipod on the iphone 3G question

Ok - I apologize if this is a really stupid question but how do I remove albums or items that i have purchased on my iphone using the itunes store? I have synced the phone with my computer and all my purchases are now on the computer but I want to remove them from the phone. I have it set where I select which playlists to sync etc. but I do not know how to turn off the seeting that syncs the purchases to the phone. Therefore my iphone has run out of memory space and I can no longer make any additional purchases. Thanks -