[michaelt]

You jam at a slightly different frequency - different enough that you can tell the signals apart with your hackrf, but close enough that the receiver chip can't [1]

Of course, the strategy I've heard outlined is to jam and record one rolling code, then a second one, then to replay the first code so the fob holder sees the system respond to their button press but the attacker has a ready-to-use code. If people are seeing their cars failing to unlock, it's not that specific attack.

[1] Page 63 of https://samy.pl/defcon2015/2015-defcon.pdf