It isn't, but try telling them that. The Constitution provides for one method for removing a sitting senator - impeachment.
No, you don’t impeach Senators any more than you impeach members of the House. You impeach members of the executive and judicial branches. Sitting federal judges, cabinet officials, POTUS etc.
The way a senator can be removed is by being expelled from the Senate by the Senate, or being compelled by circumstances to resign in shame/disgrace.