Month to month contract agreement

They made a month to month contract with me. However, when I cancelled after more than a year and a half, they refused to acknowledge that a month to month contract even exists with them, even when I sent through a copy of a signed contract clearly indicating that it is a month to month. They sent me a balance of contract amount of almost R1500, after me following up several times with proof, they swept it under the rug and kept billing me R1500.