It mght be sticking due to a bent sheer pin or its dug into the prop or both. I have had some tough times removing them at times. Mines a motor guide but I imagine Minn Kota has a sheer pin as well. It's going to take some prying, tapping, cursing, and praying and possibly paying when all is said and done.
I have had to actually cut one off before after I had all I was gonna take.... Found that original owner of the trollling motor installed a homemade pin out of a nail, and that puppy was never gonna sheer off. Hopefully that's not the case here. Keep working it and good luck.