Former employer refuses to cancel work permit and give release letter

Hello,
After my Chinese employer issued a work permit and emailed it to me to get Z visa. I decided to withdraw from the offer and not get the Z type with them. I got another offer in another city and wanted to go with them but it turns out that the new employer can't apply for work permit for me before the former employer cancels their work permit. I asked them to do so but they refuse. Is there anyway for me to cancel this work permit letter myself?
Thanks