I had the same problem on mine, when I was in the process of transfering the O2 sensor from my oem manifold to my dc one. I sprayed some rust remover/loosener on it first. I had the wrench on it, and I put my weight on it with my foot, and after a few light hops/kicks, it came loose. Then I transfered it with the O2 extension that came with my DC and it was fine. It was funny, as I tried to have a shop take it out too and they couldn't.

My friend covered the wrench with a shop cloth and started to pound away with a rubber mallet and that worked for us when we worked on his car.

Might want to try that.