I while back I made a spray paint "booth" it was pretty much just cardboard boxes arranged to block the wind. Even though I was pretty impressed with my set up, there were several gaps in the booth and I ended up with bright teal spray paint on the patio.
This was not ok and I was really worried about our deposit. So I googled how to remove spray paint from cement. After reading about sand blasting over and over I finally found one that said to use oven cleaning spray. At first I thought yah right, but I really didn't want to have a rent a sand blaster either. So I gave it shot.
What do you know it works! GREAT! Spray it on let it set for 30 min or so (I lost track of time so I can't really say how long it was there) then give it a little scrub and spray clean, it is amazing.