The last owner probably used "BetaLink" glue. It's what BMW said to use. A lot of guys have used 3M double sided adhesive - that is what I would use. No way that stuff(Betalink) is gonna come off. You will need to have a body shop cut off the old spoiler, paint the trunk lid and then install a spoiler(of your liking). Sorry for the bad news.
You might also want to have your mirror caps and front bumper painted while they are at it. Normaly wear and tear from rock chips suck. Nothing like some fresh paint to jazz up your car.
