Before beginning the cancellation process, it is important to understand each Thai Airways cancellation policy. Check the policy given below and you don't have to face any kind of trouble.
Cancellation of flight tickets within 24 hours of booking is free of charge.
After the flight is over, the airline will give you the opportunity to collect the full compensation.
If you cancel your flight within seven to ten days of your scheduled departure, you will be refunded less the ticket price.
If your flight ticket is canceled by Thai Airways, you can claim a full refund.
To claim a refund, Thai airways cancellation policy says you need to fill out a refund form from the official website or by calling a live support person or at the airport.

Flight cancellation through official site

Open the Thai Airways official site.
Click on Accept button.
Tap on Book & Manage option.
From the first row, click on Manage My Bookings button.
Click on Enter Booking Details and sign in with correct login credentials.
Here, find your reservation and click on Cancel Flight button.
Follow the rest of the on-screen instructions and pay the required charges.