I believe it is the Standing Rock tribe but I could be wrong. Bottom line is there were 3 different Sioux tribes the NCAA required them to get consent from, UND got approval from 2. The 3rd tribe had an elder on the council who refused to bring the matter before the tribal council to vote on. So it's not as if the 3rd tribe voted "No", rather there was never any vote taken on the matter at all. Polling of tribal members indicated a majority in favor of the name but it didn't matter, no official action taken to approve the name, no go. NCAA said, sorry 2 out of 3 isn't enough. The NCAA did not make UND drop the name, instead they put them on the "hostile and abusive" list and UND could not host any NCAA tournaments, could not wear the logo on their uniforms in any NCAA championship event, other "enlightened" schools refused to schedule them, etc. The short of it is it made UND's life hell, they had no choice but to drop it.
Where Sioux nickname supporters get pissed is the double standard UND faced as opposed to FSU, Utah, etc. The standards they had to meet were much higher than those cases because, you know, money.