Noun

1. a rejection, a vote against admitting someone
2. 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.

3. the act of so rejecting someone

Verb

1. to vote against, especially in an exclusive organization

   If you're not from a moneyed, well-connected family, you can count on getting blackballed from the fraternity.

2. to ostracize