Resignation not accepted even after completion of notice period

Sir, I gave my resignation from PSB on 20.1.17 mentioning to serve my notice period of 3 months upto 20.4.17 and after few days I received a letter from HR dept. stating that I cannot assume being relieved even after completion of my notice period till I get an acceptance from competent authority. I performed my duties well during the notice period. I got hospitalized on 31.3.17 and was on medical leave up to 19.4.17. I joined bank again on 20.4.17 to get relieved finally. But to my surprise I got to know that even after several telephonic enquiries and reminders, my resignation letter was not sent to top management for their approval on request of my branch head. My branch head has sanctioned somw loans without my consent and got the documents executed by borrowers and the same was pointed out as an irregularity in audit reports because of which he asked me to sign the recommendation later. I never inspected those accounts and they were not processed by me so i refused to aign and just completed the other tasks as per my office order before leaving the branch and stopped going to office after that. HR dept says that to get approval of higher authorities, my branch manager's recommendation for my resignation is necessary so they are not even forwarding my resignation to higher authorities even after completion of my notice period. Now my manager threatened me that if I do not sign those files then he will not let me get relieved from bank, as he would be issued a memo for financing single handedly and so he his not giving his acceptance for the same and has aaked me to come back and do the compliance of other work also(which is of routine nature and could be done by other officers too), apart from signing those files. Please guide me what to do.