is to die on office. In the absence of death, there is resignation. Then there is the most common method of removing a member - by voting him/her out of office.
Finally, there is the least common method - a vote of 2/3 of the sitting members can expel a member.
The Constitution is pretty clear on the requirements for holding office and there are no provisions for recalling a member or for term limits.
It's less clear if a previous impeachment & conviction can bar someone from serving as a member of congress. For example, Alcee Hastings (D-Florida, 20th District) was a federal judge who was impeached by the House and Convicted by the Senate in 1989. As a result, he was removed from his judiciary position. The Constitution says that the consequences of impeachment are limited to removal from office (which he was) and a bar from holding future office (which the Senate did not specifically vote on, so he was not prohibited from being elected to/serving as a member of the U.S. House of Representatives).
http://en.wikipedia.org/wiki/Alcee_Hastings
From the Wiki Article: "In 1988, the Democratic-controlled U.S. House of Representatives took up the case, and Hastings was impeached for bribery and perjury by a vote of 413-3. He was then convicted in 1989 by the United States Senate (also controlled by the Democrats), becoming the sixth federal judge in the history of the United States to be removed from office by the Senate. The Senate, in two hours of roll calls, voted on 11 of the 17 articles of impeachment. It convicted Hastings of eight of the 11 articles. The vote on the first article was 69 for and 26 opposed,[3] providing five votes more than the two-thirds of those present that were needed to convict. The first article accused the judge of conspiracy. Conviction on any single article was enough to remove the judge from office."
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