Bad billing by nxatel

I took a contract with nxatel i 2013 and in 2015 i completed the contract and cancell it R600 p/m...but woow suprise..in 2017 december they start billing me..for what?i reall dont know and their debit order cant be stopped..the take whatever ammount they like...R300,500,450....i have in contact with them for so long now and no one can explain to me whats going on the..but the billing is continues....anyone who can help me please do...i will be happy to nail them..