There are theists that claim to embrace science and attempt to use it to support their arguments and those that don't. If you are dealing with the latter type, then I would expect responses as you have provided. In that case, anything goes and there is no way to win the argument because they share virtually no assumptions with you.
But I do think you can make ground with the former type. If debating one of these, then there is definitely a reason to prefer one unprovable hypothesis over another where one conflicts with science and the other does not.
"I certainly regard myself as one of the former type," says Theist, slightly miffed. "I make no claims contrary to reason. I'm as interested in evidence as the next person. I'm not claiming the multiverse hypothesis is necessarily wrong, merely that there's no evidence favoring it over a Creator hypothesis. At least, not yet. I'm keeping an open mind. (Which is more than can be said for many in the atheist camp, I'm afraid.)"
"As to unlikelihood," continued Theist, "well, sure, the idea of an incorporeal, all-powerful Creator strikes us as unlikely. But how could it be otherwise? Of course we mammals can't comprehend it. It's utterly outside our frame of reference. The astonishing thing would be if we, who only started growing cerebral cortexes the day before yesterday, could
"And if twentieth-century science teaches us anything, it's that a hypothesis can't be ruled out merely because it's counterintuitive to us smart apes," said Theist. "Even Einstein didn't accept the implications of quantum mechanics. If he could continue to doubt to his dying day even in the face of mounting experimental and theoretical evidence, surely I'm justified to suspend my judgment on this matter until actual evidence should appear,"
That is certainly how we evalute testable hypotheses. What is the statement "Extraordinary claims require extraordinary evidence" if not a reason to prefer one explanation (the one that is consistent with science) over another (the one that isn't)? Why should the rule be any different with respect to untestable claims?
"With respect, sir," nodded Theist, "that is exactly the point: We have no evidence one way or the other, at least not yet. As of now we only have competing hypotheses, neither of which is ruled out by the available evidence."
A claim that violates no scientific principles is more likely than one that does because the latter would invalidate all our scientific progress to date while the former would not. Isn't that reason enough?
"I'm not convinced a multiverse is necessarily more plausible than a Creator," replied Theist, smiling. "But I'm willing to entertain the possibility. Like the man said, truth springs from argument amongst friends."