Global warming stopped [insert random number of years] ago is probably the most common and mainstream of all the climate denier arguments out there. It is shallow and wrong and there are many refutations of it out there. I think this video from Skeptical Science is about as clear and straightforward as it gets.
If someone watches that and still thinks the argument holds water, then they can not be reached with reason.
If attribution arguments are too complicated, there is always this excellent graphic:

(click to enlarge)