Choose from following options,
On Net banking:
On Mobile Banking:
You can also send an SMS from your registered mobile number
SMS KCCLOST(space)(Last four digit of Credit Card number) > 5676788 (Standard SMS charges applicable).
If you are unable to block your credit card through above steps, you can also visit the below link to contact us and immediately block your credit card: https://www.kotak.com/en/customer-service/contact-us/phone-banking.html