A debt collector is making threats to ruin my reputation, is this legal?

You just have to tell him to stop. Under the Federal Fair Debt Collections Practices Act (FDCPA), collections agencies and debt collectors:

• May not communicate with any third parties (anyone other than you or your attorney, including your boss and family), except for the very limited purpose of locating you, unless you give the debt collector permission to speak with third parties.
• May not call you at unreasonable times, which is usually defined as any time after 9 p.m. or before 8 a.m.
• Has to stop all communication if you tell them in writing to do so.
• May not engage in any contact that is intended to “harass, oppress, or abuse” you.

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.