Hero Fincorp — Collection harassment

I had taken a maestro edge from hero moto corp on loan years ago and had paid all my emi's however due to a technical glitch from hero moto corp side, 1 of my emi didnt get debited as my account was on NACH, which i eventually paid manually couple of days later. Now they are harassing me every time and threatening me that they will not give me my NOC even after paying all the emi's as there is 2 bounce charges on my account(rs1077 in total), 1 was because of me which i agree to pay after they rectify the charge and 1 because of their technical glitch (no emi is pending only bounce charges). I have explained the same thing to all the rep who calls me again nd again l to recover that bounce charges, now im fed up of their harrassment and im seeking some help..