According to Article IV, §8 of the Wisconsin Constitution:
Expulsion is probably a bridge too far, given the 2/3 hurdle, but, at least, Republicans should consider some form of censure for the contemptible behavior of the missing Democrats.
Eschewing expulsion, I simply wonder aloud as to the possibility of declaring the Democrats to have abdicated their offices. Article IV, §14 states:
Filling vacancies. Section 14. The governor shall issue writs of election to fill such vacancies as may occur in either house of the legislature.
How long must a member be absent before his seat can be deemed vacant? The maximum required period for eviction in Wisconsin is 30 days - perhaps the governor should begin sending out notices.
UPDATE: The "Badger 14": Escape from Wisconsin is a new blog about "why and how the "Badger 14" -- the Wisconsin senators who vacated their seats and fled the state -- should be replaced with new senators."