They can only be removed from office if they are impeached by a majority vote in the House and convicted by 2/3 in the Senate.
Justices serve lifetime appointments. Under the Constitution they can be removed from the Court only by first being impeached (accused) by a majority vote of the U.S. House of Representatives and then convicted by a two-thirds vote of the Senate. There is no precise standard for determining whether a justice has committed an impeachable offense, though the consensus is that removal should be for criminal or ethical lapses, not for partisan political reasons. No justice has ever been removed through this process, and only one justice of the Supreme Court has ever been impeached. In 1805 Justice Samuel Chase was impeached in the House by his political enemies, but the Senate failed to convict when it became apparent that Chase’s opponents were after him not because he had committed any wrongdoing but because they disagreed with his decisions. The possibility of impeachment may have been a factor in the resignation of Justice Abe Fortas, who left the Court in 1969 after allegations surfaced that he had accepted a questionable fee from a private foundation. Some conservative groups rallied for the removal of Chief Justice Earl Warren in the 1960s, but their efforts failed.
http://encarta.msn.com/encyclopedia_761574302_2/Supreme_Court_of_the_United_States.html