Branded search terms cost way more for competitors for reasons I can't be bothered getting into, most likely they are losing money by doing it. You can counter it by bidding yourself - which on branded search terms is usually dirt cheap. But ultimately, the only real counter is delivering a better service.

Place ads for their brand that links to a negative review about them?

Try to drum up public outrage to get them to stop? Maybe if they use your trademark there's cause to sue? Otherwise good luck.