Denaturing scope rubber

[Gloftoe]

I had a heat gun that did the same thing. I ended up just wrapping the whole thing in electrical tape. It won't last, but nothing else would reliably do the trick.

[SLICK_N_STICKY]

Goof off

The Miracle Remover less than $10 bucks

[JakeGraves]

Acetone or MEK might work but you risk it eating seals and such if you get it on them.