contest pig
n. Disk jockey jargon for a listener who tries to win every phone-in contest by utilizing speed dialers and multiple telephones.
Eliminating the first caller cuts down on the ‘contest pigs,’ the people who call all the time," says Dean Anthony, program director at WKJY's AM sister, WHLI / 1100 AM, where they’ll be giving away weekend trips later in the summer. "We also have a rule: If you've won in the last three months, you can't win again. There’s a whole group of people listening to all the stations, figuring out when a contest is going to be run, and they’re always there trying.
—Terry Kelleher, “The dial’s hot with summertime contests,” Newsday, June 28, 1995
Within the radio industry, they're known as "contest pigs."

It's an unflattering term for people who can be the bane of a promotion department's existence. They constantly enter station contests, devising elaborate ways to beat the contest system. Using false names and speed-dial telephones are the most common techniques.

"Contest pigs" also are mercenaries. Interested more in contests than stations, they continually jump around the dial looking for contests. Promotion directors see the same people winning contests at all the stations.
—Kevin Brass, “Radio stations seek to outwit 'contest pigs,' keep real fans,” Los Angeles Times, August 10, 1992
1986 (earliest)
Sometimes only a limited number of listeners play the contests. Some people, called "contest pigs" and often known by name by those in the industry, tune from station to station playing the contests.
—Neill Borowski, “Playing Games For Listeners Radio Stations Hope Contests Will Pay Off,” The Philadelphia Inquirer, November 03, 1986