1. Ensure you have a copy of your letter of resignation with you.
2. Your 30 days notice period as per your contract is validated whether your contract is open or for a set duration.
3. Your employer has no right to keep you working beyond the agreed upon resignation period.
4. Issue a letter in good faith to your employer based on his verbal decline of your resignation and respectfully remind him that the contract agreed and signed between you both clearly indicates a 30 day resignation period. As a result, thank him for your tenure at the company however do state that your last day shall be x day and that you would appreciate receiving all your entitlements as per contract etc....gratuity, final settlement etc..

If the employer is still proving tough to give you your rights, then a trip to the labour dept will be required I am afraid......

One always tries to avoid the final course of action to avoid drama but you may be compelled to take this route if your employer fails to deliver your legal rights.