These people are ignorant! If they totally ignore hadeeth, they are apostates!
One has to refer back to hadith to understand and follow the Sunnah of the prophet SAW which is not in the Quran, moreover, Quran provides us certain sets of instructions, the procedures to carry them out can only be found in hadith. E.g. Quran says to establish prayer and pay zakah, now we have to go back to hadith to understand how to perform salah and the method of paying zakah.