Sometimes it's possible to dispute the claim, but if you haven't done so by now, that timeframe is likely out the window (sometimes 15-30 days). If the surety hasn't heard back, they'll either investigate themselves, or in the case of smaller claims, they'll just pay it out- they know they're going to get their money back one way or another. It creates an ugly catch-22. Without your bond, you can't legally contract, and you need the money from contracting to pay the claim. Contractors need to cut it off at the pass and dispute client claims if they are without merit.
That is the biggest problem that many contractors have- they don't dispute the claims made by clients. Get out there and defend your business and your good name, contractors!