NOC and Resignation

Sir/ Madam Am working for Indian Air force (IAF)and my probation will end at August 2016. I had applied for NOC at April 2016 to write ISRO exam but they did not approve my NOC saying that 'Am still in Probation and should apply for any other exams only after completion of Probation'. I wrote ISRO exam in April without NOC. After getting selected in ISRO, I wanted to resign in IAF but if they do not approve my Resignation. What should be done as 'IAF is not liking to relieve me'. Please do the needful.