@Jef *******
thank you for your helpful reply. This is a new question.
It appears the key is how you got the DTV visa.
Note the different instructions. If you obtained it from a Thai Embassy, such as Sydney Australia, contact them. (DTV Visas were issued with a Pasted Thai Visa Certificate into your passport).…then with the beginning of the Evisa online application system, the Thai Visas are now ISSUED by email notification.
So...the question is....
Who do you contact to cancel the DTV visa ?
From your reply, contact the Thai Embassy or Consulate where you applied for the DTV visa.
Thanks again...