No one can legally "garnish" SS payments.  They can be withheld only for taxes, student loans, and certain court ordered payments such as child support.

Criminals masquerading as debt collectors attempt to extort money from people by scaring you into believing that you will be criminally charged (they use the bad check or hot check claim a lot) and go to jail, lose your driver’s license, have wages garnished without actually being sued or going to court, be sued, and a variety of other variations on this, all for an alleged or nonexistent debt.  One of the tricks they use is to call your family, friends, neighbors and/or places of employment (past or present or both) to create panic and embarrassment so that their intended victim calls them and they can scare that person into paying their extortion money.  They will use the words “Mediator”, “Arbitrator”, or “Law Office” to make you believe that they are something other than a debt collector want-a-be and that they don’t have to follow Federal and State collection laws.  They do have to follow those laws, but they won’t.  They also use the “process server” ruse who calls and claims he is going to serve you, but then says you could avoid it by calling another number where they will ask for money to “make it go away” (this is actually the same place, they work in teams, one pretending to be the server, and the other usually pretends to be a lawyer).  Process servers do not ever call ahead so that you can dodge them.  Process servers get paid to serve papers, nothing else.

Federal law (FDCPA) requires them to send you a letter (US MAIL ONLY) postmarked within 5 days of their first contact that contains their name, physical address, the creditor’s name, and the amount of the alleged debt.  It also must contains “mini-Miranda” telling you that it is an attempt to collect a debt and that all information will be used for those purposes.  The one other important thing that this letter must also have in it is that you have a right to dispute the debt within 30 days of receipt of the letter and if you do so, all collection activity must be stopped until the debt is verified.

Read up on your rights here and also make a complaint at this government site:  http://www.consumerfinance.gov/

Also file a complaint with your State Attorney General's office.
List of State AG’s offices:   http://800notes.com/faq/attorney-general