"The Freelancer may terminate a Fixed-Price Contract at any time if no payment has been made. If a payment has been made on a Fixed Price Contract, the Freelancer may terminate only with the Client's consent or after the payment has been refunded."
As a freelancer I've closed several contracts recently, and at least one or two have been fixed price (in fact, the most up to date one on my profile, as of today, is one). These are old "morbid" contracts left open to fester. I would have contacted the client at least twice and then given them notice that I would be closing. then after a couple of weeks, closed the contract. No feedback from the client, and it looks as if they've just left the platform, either permanently or temporarily.
Anyway, the point is, money had been paid, and I did close the contracts, no problem.