I had trouble removing my cog to change it out for a 17t. This is what I did; I used a large screwdriver & hammer to bang it loose, then a large pair of channel locks to loosen it all of the way.

I rotafixa'd it on & it wasn't budging. I even did trackstands, rode & skidded w/o a lockring but it was on there good. Trust me, a 3lb mallet & screwdriver are your friend if you're not planning on reusing the cog. Just be careful.