Negative remark at experience letter

Hello All, I stuck in a situation where my previous employer has added remark at my experience letter as "breach of undertaking given to employer during deployment on customer project." I worked with them for more than 8 years received awards, client appreciations and good ratings all the time. I served notice period and abide to all process defined by company. While working with one client this company asked me to sign one paper on which they mentioned like I can not join client or any comparator company while working with this client. This was not written on bond paper but it was on company letter head. Management forced me to sign this letter if I wanted to work in this project. I requested them to remove this remark as it will mess my career going forward. However they haven't replied on it. Thank you in advance for all your support.