I believe you can use fishing line to saw across the glue on the back side. Clean up with some solvent/goo gone to get the excess off. I think they just use a weatherproof double-sided tape/foam to stick them on.
I moved my emblems around and removed them using fishing line, but be warned it will leave a lot of residue behind. I used bug and tar remover followed by Adam's Polishes Revive Polish to remove all the residue. It took a bit of elbow grease to get the trunk lid clean, but I think it was worth the work. Here's a photo of the finished look: