I had a similar problem. I bought a little wiper arm puller off eBay. Not the best tool I ever bought. In fact it was a crappy POS, that needed to be ground a bit to fit...but it DID pop the arm off. This is the tool I used:
Great Neck 25319 Windshield Wiper Arm Puller Brand New | eBay
There are also much better tools one can buy for this job...but this is a cheap solution. I needed to hit this one with the die grinder a bit to make it work on the E53. Here's an example of a much more upscale set:
Wiper Arm Puller Tool Set Fits BMW Audi Merc VW GM Peugeot Ford Great Set | eBay