my mate did this once
get some duck tape and go to where the spray cans are and quickly turn one on and tape it so it stays open, then light the spray with a lighter and put the can back on the shelf and run
(What my mate didnt tell me is he did two, but he turned the other one round, That night it said said there had been some mini explosions at the local Asda)
Credit to Pity for Awsome Sig