The correct response to these supporters is: “How do you know that the polls are rigged?” — it’s a very significant question.
Whenever these kinds of wild assertions are made — the very first response must always be “How do you know that?” — because if someone can tell me how they know something, I can check their sources, and then I’ll be convinced. But when someone just tosses out a comment like “The polls were rigged” without any explanation for how they know that — then my reaction to that is going to be disbelief.
If they’re just guessing that Trump has more support than the polls are saying — then they’re also just guessing that the polls are wrong. How can anyone *know* how much support he has without…um…running a poll? So either:
a) Run a demonstrably-not-rigged poll and compare the numbers that come back to the other polls
b) Find some actual evidence of the rigging (under-cover payments to polling companies or whatever)
Neither of those things seems to have happened.