So, what was done with verses like:
“And whatsoever the messenger gives you, take it. And whatsoever he makes prohibited unto you, abstain from it.”
“Whosoever obeys the messenger has obeyed Allah.”
“And we have sent down The Reminder (the Qur’an) unto you, (Oh Muhammad), so that you may explain to the people that which has been revealed to them.”
“And by your lord, they shall not believe until they make you the judge for whatever arises between them, then find no uneasiness in themselves regarding what you have decreed.”
Et cetera, et cetera, et cetera?
For that, there were a few things, so, depending on which resource/Quranist you spoke to, he'd probably give you some mixture of the following:
I knew a lot of people who were quick to point out that Mohammed wasn't infallible in his behavior and so to emulate him could not have been completely reliable, and they would typically point to 66:1 to make that point. I knew others who, if you threw those verses at them, they would simply combat your verses with the ones in support of the Quran alone--ones like what Dr_sloth was alluding to, the popular ones being 6:114, 77:50, and basically all of the ones that speak about how perfect and complete the Quran is and how legislation is for God alone.
Now, there were others, and this is kind of where I was at the time, who didn't really even bother with much of that, and who went right to the logical trouble with the hadith: that it was a glorified game of telephone compiled far after Mohammad's death, that many of them were simply not true (I'd point to ones with bad "science" most of the time), and that they were far too unreliable to ever use as legislation. Many would say that Mohammed's rulings at the time were supposed to stay at that time, influence the people and community of that critical time--otherwise they would be in the perfect and complete book, and not in a mixed bag of questionable and sometimes contradictory accounts well after the fact.
That was the argument, anyway.