Worst medical-aid of the year

On the 1st of October i informed Bonitas that i'm retired and can't afford the current medical option anymore and need to downscale to a lower option. After numerous emails which they did not respond to, i decided that it will be better to terminate my membership with Bonitas. On the 15th of November, i yet again inform them that i'm terminating my membership with immediate effect. They responded by sending me some sort of termination form which i completed and returned to them. They acknowledge receipt of my termination and their last contribution were paid on the 2nd of December. This month, i received a statement from them that i still owe them for December, although my membership were terminated at the end of November and use of the medical aid as such were terminated,and because of this, i had to pay for my prescribed medicine myself. Now, they want me to pay for something i did not and could not use.