Proposition 89, voted into law by Californians in 1988, gives the governor the authority to veto the parole board's positive recommendations in the case of people imprisoned for murder (see the end of the Parole Process document on the website of the California Department of Corrections and Rehabilitation for one current mention).

In addition to the sources listed in the answers above, you might want to look at the following articles:

• "Wilson Accused of Manipulating Prison Board" by Noel Brinkerhoff
  State Net Capitol Journal (California), May 3, 1999

  [excerpt]

  "In a Los Angeles Times story last month, Davis said those serving life sentences, including second-degree murder, should not be paroled. 'If you take someone else's life, forget it,' said Davis." [unfortunately, I can't find the original LA Times story]

• "Governor 'hard-pressed' to find justification for parole"
  Associated Press State & Local Wire, April 9, 1999

  [excerpt]

  "The Los Angeles Times reported that Davis, exercising his authority under state law, had overruled the Board of Prison Terms' unanimous recommendations to parole five prisoners convicted of second-degree murder....'The governor feels very strongly about this,' [Davis spokesman Michael] Bustamante said in an interview. 'He'd be hard-pressed to find any reason why someone who's committed murder, even second-degree murder, should be paroled.'

  "Asked whether Davis was ruling out all paroles while he was governor, Bustamante said 'there may be a circumstance,' such as strong evidence of innocence, that would change Davis' mind. But he said Davis was adamant that 'if an individual commits crimes such as murder, that person should spent that time (life) behind bars.'"

• "Davis denies parole for battered woman; Criticism from advocates for victims of abuse" by Bob Egelko and Kelly St. John
  San Francisco Chronicle, February 16, 2002

  [excerpt]

  "Gov. Gray Davis has denied parole to a woman who murdered her boyfriend, rebuking a ruling by his own appointed parole board that she should be freed because she was a battered woman....Since Davis took office in 1999, he has approved just one of 96 murderers recommended for release: the case of Rose Ann Parker, who was convicted of killing a boyfriend who threatened to kill her and her children.

  "In three other cases, Davis vetoed the release of convicted murderers who the board recommended for parole because they were battered women.

  "Critics, including some fellow Democrats in the Legislature, say Davis has imposed an illegal no-parole policy, citing his record and a 1999 statement to reporters about parole -- that 'if you take someone else's life, forget it.'"

• "Inmates Fight Governor's 'No Parole' Policy" by Bruce McClean
  Contra Costa Times (California), July 25, 2000

  [excerpt]

  "[Randall K.] Cowans and fellow inmates are fighting what they and others allege is an unwritten and illegal 'no parole' policy carried out by Davis and the parole board, stacked with former law enforcement officers or those with close law enforcement ties.

  "They're not alone. State lawmakers, attorneys and even a judge have joined Cowans and his fellow inmates."

• "Speaking for victims of domestic abuse: Woman who killed her abuser spreads messages of anti-violence to students" by Brooke Edwards
  Daily Press (Victorville, Calif.), April 13, 2008

  [excerpt]

  "Though she doesn't paint herself as a victim, [Flozelle] Woodmore said fear and a lack of options were what led her to fire a single shot and kill her abusive boyfriend when she was 18 years old....Given her good behavior and the circumstances of her crime, the parole board recommended Woodmore for release when her 15 years were up in 2002. However, then-Governor Gray Davis overturned the decision, refusing to allow any convicted murderers out on parole.

  "Woodmore was recommended five more times before Gov. Schwarzenegger upheld the parole board's decision and signed her release last August."