it REALLY all entirely depends upon the adhesive used, it is Polar or non-polar?
everything has a solvent (yes even Diamond!).
personally I always start with plain hot soapy water, and it that fails I go to WD-40 (it has a good mixture of different solvents in there), then move to the Harsh stuff if that fails such as Acetone (nail polish remover if you have no pure acetone).
if THAT fails there`s always the "Brute Force" tactic, stick in the freezer for an hour, take it out and use a paint scraper!
and as the saying goes, if at 1`st you don`t succeed, destroy all evidence that you even tried