The nuts rust to the stud coming out of the joint. I have been successfully clamping a vice grip to the back side and spinning the nut off. Just did it on a 2008 Mazda 3 that my daughter drives (yeah, they were rusty). They can be a pain. If the vice grips don't work, heat the nut up and hold with vice grips. Heat up, hold with vice grips use impact. Worst case scenario is you cut it off with a grinder, but I've always been successful with the vice grips.