what to do if someone else is using my Social security number?

You need to report that to the office and police

The only thing that Social Security can do is lock your on line account. That way no one can use it to file for benifits for you. IRS also has a repot in system that you can lock that as well. It's a pain to have to do everything in person....but a bigger pain to have to clean up a mess. You really want to lock your credit rating by getting in touch with the credit bureaus. No one can create new credit for you if you lock those accounts. If you are already a victim contact your local police agency.