A debt collector is threatening to ruin my reputation. He says that he's going to tell my boss and my family that I'm a cheat and deadbeat who can't be trusted. He's also been calling me day and night. How can I stop him?
You just have to tell him to stop. Under the Federal Fair Debt Collections Practices Act (FDCPA), collections agencies and debt collectors:
So the debt collector should under no circumstances call your boss or family to tell them you are a deadbeat, and should not be harassing you; those sorts of phone practices are specifically made illegal. You can further tell them (in writing) to stop all calls.
Of course, they can still go ahead with other legal remedies, including potentially suing you—you can't make them give up on collecting a lawful debt. But you can force them to do it the right way, by the numbers, and not be making threats or harassing you.
If a debt collector is harassing you or violating the rules, one option you have would be to report them to the Federal Trade Commission (FTC). You don't mention your state, but many states also have government units or agencies that will look into abusive debt collections practices; they may also have their own fair debt collections laws.