Back when US senators were chosen by state legislatures, the older states had provisions in their state constitutions that provided for a state legislature to impeach a US senator and replace him. This has never happened because the courts have ruled that the US Constitution does not provide for the impeachment, recall or removal of federal officeholders except through election, and Article VI of the US Constitution, i.e. the Supremacy Clause, makes the US Constitution superior over all state constitutions.
Each House of Congress can expel a member by a two thirds vote, which has been done in the past.