Tell them straight up. Please don't block the driveway. It's in my handbook too, but people do it anyway so if you have to verbally tell them also, then do it. I can't tell you how many times my husband comes home and can't get in the driveway because someone has blocked it. He would park on the road and go back and move his truck later. One day a certain parent blocked the driveway- frequent occurance- and he came in and asked her to move it so he could get in. She was a bit embarrassed but did move it. i revised my handbook- no parking in the driveway AND don't block the driveway.