Non payment of salary for notice served

I worked in a recruitment consulting firm for 7 months, which is a start up company. While my resignation was accepted the owner of the company introduced a Non disclosure agreement with an estamp for data security. It had a clause, which prohibits any employee from joining any competitors or clients for 3 years, which I don't find appropriate to sign, upon refusal from signing he did not release my salary for 41 days and experience letter, stating an investigation will be and standard policy is 45 days from reliving date. Post my release I got an offer from one this employer's client, with whom I never interact during my services in the company. Also my fiance works here, so he referred me and knew this company before joining the consulting firm as I previously working with its sister company. After which the owner of previous company is not ready to give my salary and experience letter, also threatens to file a case against me. I did approach Labour Department on the same, but no good came out of it, they said it's a lengthy process and your salary does not come under 18k bracket, so nothing much can be done. It's not just me this owner has not paid few of my other colleagues too. Really appreciate any guidance and help on this matter.