Setting up a use voting table
If you want to restrict your votes to "one user can vote for another user once", you simply use the two user id:s as primary key. As for the vote, just use one column and make it an enum:

CREATE TABLE IF NOT EXISTS `user_votes` (
  `voter` int(11) NOT NULL,
  `voted_for` int(11) NOT NULL,
  `vote` enum('up','down') NOT NULL,
  PRIMARY KEY (`voter`,`voted_for`)
)