How can I stop a creditor from calling my work, he is threatening to tell my employer my situation

He CANNOT tell your employer (in the US), that's against the law. For that matter the THREAT to call your employer is against the law.

Tell "him" that and then report the agency to your State's Attorney General.

Tell him to STOP calling you and if you can get an address, send him a letter telling him to STOP calling you.