Sign-on bonus repayment

Hi, Recently I left a company due to internal cultural, ethical issues after 10 months of service. I signed the offer with a sign-on bonus of 12 months of service. The HR has reached out for full amount repayment. I told them that I am fine to repay the prorated amount, but didn't hear back anything. Do you have any suggestion here? Much appreciate your help in advance! Thank you!