I have no idea whether this would be safe to use, but just in case...
A couple of years ago I bought a stainless steel, one-sided, food grater that had a massive sticker adhered to its flat side. Nothing I tried would remove it - and apparently a lot of people had this problem.
I went to the company's web site and found a note addressing the issue. They said to put a little olive oil on it and let it sit for a while, then rub it with a dish sponge/cloth.
I had to apply the olive oil several times, rubbing the surface between each application. After a few tries the sticker/glue was completely gone.
Again, this was on stainless steel - not soft plastic.