Definition of blackball:
(n) :
a
rejection, a vote against admitting someone; a black ball used to indicate such a negative vote
"regardless how many other people may have voted to approve a candidate for membership,a single blackball will reject the candidate."
(v) :
to vote against, especially in an exclusive organization; to
ostracize
"if you're not from a moneyed, well-connected family, you can count on getting blackballed from the fraternity."