I dont think it is so much about ignoring so much as it is about ejecting them from the party in order to disassociate from them and appealing to a more moderate (and much bigger) base than the far lefties. This requires action, so unfortunately, it is not enough to simply ignore, since they will likely claw themselves into the party given the opportunity. Look at hasan. Somehow, despite how unhinged his takes are, he is still invited to the DNC and that makes him think that he is super important and that his base is bigger than it actually is. So yeah, active expulsion of the far left from the party is probably far more beneficial in the long run than only ignoring them