The main thing is heat.. so gotta use a heat gun to basically melt the adhesive, while running a thin wire in between. Have heard of some who have used fishing line for this and got good results. It is going to be a long a slow process, so you gotta be patient. If you rush it, you're gonna remove more than just the glue.
If you've had the spoiler on for long though, there's a chance you may already need to paint it anyway, since the colour fade would be obvious.
P.S. Not sure if hair dryers would work as a heat source.... I do know of some who've removed tint adhesive using hair dryer, but it wasn't thorough. They still had to scrap some of the bits that still remained after. Might be worth a try, if you don't wanna buy a heat gun just for this one off.