Facebook has an integrated feature to block Facebook Chat friends, so if you want to block any of your Facebook contacts from Facebook Chat then just follow the method written below.
Block Friends on Facebook Chat
1- Login to Facebook
2- Navigate to Accounts >> Privacy Settings
3- Click on Block List option
4- Here enter the Person name, or Email address to whom you want to block and click Block button and that’s all
Your blocked friend will see you in Facebook chat but they will not be capable anymore to interact with you.