deduction while im not a member

I have been employed at city of Tshwane in 2009 as a cashier and I joined samwu union, in 2011 I resigned at Tshwane municipality, when I was appointed at department of agriculture. in 2012 I was appointed by city of Tshwane as permanent employee on 12/04/2012 to date, during the induction we were given forms to complete for unions and I completed samwu membership form. To my surprise imatu union has been deducting R62.00 every month from my salary since May 2012 until November 2014 and I haven't joined , I have been communicating with Aneela Ramdin and she told me that its possible that someone could have filled the form on my behalf, I told her I didn't join imatu and she insisted that I have joined in July 2012 and provided my mothers details as a beneficiary. I have not completed any imatu membership forms, I have requested her to refund all the moneys deducted since 2012, and she said she cant because I have joined, I have also ask them to provide me with the membership form that I have signed when joining the union and they couldn't provide me with the form. can you please assist so that I can get my refund. thanks Beauty