Unauthorized purchase on my account

I just found out that there was an unauthorized purchase of R4689.90 on my account that was done at Edgars High-veld Mall on 11 October 2019. i went to the store to enquire about it and i found that the signature is not mine. They asked me to go make affidavit from police station and comeback to log a dispute, when i come back the sales lady made me sign a blank form and said i must go home she will do the rest. I told her i don't like to sign blank forms then she said she is busy that is the only way she can help at that moment, i am not happy with the unauthorized purchase and i am also not happy about the way they handled my query.