Tell them to go ahead and try. Social Security is not garnishable except for back taxes. In addition, the maximum amount anyone can garnish of wages is 25%.
Next time they call, ask for their company name and address - they have to give it to you. When you get the name and address of the company, write a letter, return receipt requested, to them that formally requests that they stop calling. Under the Fair Debt Collection Act, if you request that they no longer call, they can't call. This should be done in writing so you have a record.

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