Conferences are governed by mutually agreed upon contracts and by-laws. The conferences basically agree, in advance, upon the rules governing the potential admission of future members. From what I've read on this board, I believe that the ACC's rules state that a membership candidate cannot be admitted if 4 members blackball them. I have heard (but don't know for sure) that the SEC grants even more veto power to individual members: members can unilaterally block a college in its same state from joining (e.g., Kentucky can block Louisville from joining and Florida can block FSU from joining).