Tried heating it up with a hairdrier first?
might be worth a bash.
I had a massive A4 sized sticker attached to my windscreen once saying I shouldn't park where I was parked. Great idea that, prevent me from moving my car from the forbidden space by sticking a huge thing over the area of the screen I need to see out of.
Anyway, hot soapy water got it right off, so I'd just try that before shelling out on a cleaning product, unless you already have something.
Means you had to remove it (and therefore read it) before driving off and doing it again?
It's not like it takes you half an hour to take off a notice
Unless of course it leaves a horrendous residue..