Ikem cleaning, Power clean super concentrate

a sales lady from Ikem phoned me around June, July 2017, she said they have very good cleaning chemicals, she then raised an order and send me the 25L of super concentrate, which i open and used a little bit, even dilute the concentrate and used again, not satisfied at all. a Week later some one from Ikem phoned me, i told them i'm not satisfied with product, and they must come collect their stuff, from then on they keep on phoning me and said i only had a week to send stuff back, i told them again to come and collect their 25L drum of stuff, i'm not going to pay R 3418.00 for stuff thats not working, i've even ask if i can drop it off at their premises, they said no... they keep on phoning me every day. What can i do, i am not prepare to pay anything, for stuff that i cant use.