norman bisset debt colectors

norman bisset contacted me last year about a suposed unpaid amount to Planet Fitness Gym i paid up my 24 month contract and i cancelled it as i was undergoing cancer treatment and i could not train in any case. they continued to put through debit orders. i stopped them as i could prove to my bank the contract was cancelled. i explained all this to Norman Bisset but they continue to harras me and now they even sending me legal letters. i see i am not alone in this. whist i could just ignore them; i think i should perhaps take this legal but that will cost me further money. do these chaps have any legal case here or is this just debt colectors who bought debt at 20c to the rand who are harrassing me hoping to get a quick buck. they even resorted at a stage to offer discount off their initial offering. i am definately not paying them one cent.