Recently I closed Paytm FASTag account. It is very simple and sharing the steps here.
1. Open Paytm app in your mobile.
2. Click on your profile image (top left corner)
3. Click on "Help & Support".
4. In the next screen, select Fastag.
5. In the next screen, click on "Need help with non order related queries? Chat with us".
6. Now a chat window opens up. In this screen select "Queries related to updating FASTag Profile".
7. In the next option, select "I want to close my FASTag".
After this follow the directions asked by the chat such as choosing your vehicle number and other details. Once you complete the steps, it will show that the closure is successful.
After the successful message, you can remove your tag from your vehicle and destroy. Also, you can apply for a new FASTag from any other provider. Hope this information helps.