Actually, the House and Senate can punish their members (and compel attendance ~ yea gads, even the Executive Office of the President can't do that).
Congressmen aren't immune from criminal prosecution, are they? And Congress's Ethics Committees can't impose criminal penalties. So if a Congressmen appears guilty of a crime, it's the law enforcement agencies that should go after him. e.g. The D.C. Police, not Congress, investigated Gary Condit's possible involvement in Ms. Levy's disappearance