Roommate refuses to pay part of their bill.?

So this is our final bill for our period of living together. They refuse to pay the water bill due to “mess” me and my other roommate created when we left behind especially countertops. The thing is we cleaned the entire apartment when we left, I took photos of the whole place so we can see we cleaned all the mess.... show more So this is our final bill for our period of living together. They refuse to pay the water bill due to “mess” me and my other roommate created when we left behind especially countertops. The thing is we cleaned the entire apartment when we left, I took photos of the whole place so we can see we cleaned all the mess. her new roommate moved in that day, after we left so they could have created mess. She blocked us both on Facebook so what can we do?
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