Many States have laws that may apply to this situation, so please check your own state. The federal law on this issue is the Fair Debt Collection Practices Act FDCPA. Pursuant to the FDCPA, if you demand that a debt collector stop calling you at work, they must stop. In order to prove notice, you should mail the notice and keep a copy, send certified if possible. This Act only applies to debt collectors, not the actual creditor. For example, if ABC collection agency is calling to collect a debt owed to Visa, the law would apply. If it is Visa, calling to collect it's own debt, it will not apply, although there may be a state law that helps.
The FDCPA prohibits debt collectors from contacting you at any inconvenient time or place. Additionally, it prohibits debt collectors from communicating with any third parties while collecting debts. As such, it appears there is no legal basis for the debt collectors to be speaking with your co-workers. Furthermore, if they continue to call your place of employement after you even TELL THEM OVER THE PHONE not to call you at work again, they have also violated Federal Law. Find an attorney that focuses their practice on the FDCPA and go after the debt collector that is harassing you. For more information about the FDCPA feel free to visit my website www.fairdebthelpers.com. I wish you luck!