Are you using an O2 sensor removal tool? I tried a few different methods before I finally got the sensors out of there to weld up my down tube. First I sprayed the bungs a few days in advance, then the day before work began, then right before I started wrenching on it. I'm not sure any of that really helped tho. You might want to rent a removal tool from Auto Zone or somewhere so you don't round off the hex. Other than that, I increasingly applied pressure steadily for several seconds at a time until it finally budged. Good luck!