Parties might not be able to purge voters from their membership, but at the very least, the Democratic Caucus can expel members by a 2/3 vote. Straight from their site, rule 1 on page 6 of the pdf they have embedded there. On the face of it, she should no longer be considered a member in good standing, and should be expelled from the Democratic Caucus as a result. In order to maintain membership in good standing, Members shall not participate in campaign activities for any candidate in a partisan federal election who is not a Member of the Democratic Party. I kind of figured there had to be a way for parties to prevent just anyone from running under their party, or the Republicans would have ran deliberately terrible candidates as Democrats in races to try and rig things long ago, by this stage.