Hi MarkZhang! If you are looking to connect with our Support team, you will want to login and submit a ticket through our Support Portal. Otherwise, the community is a great place to ask questions to and interact with other Qualtrics users!

For Mark or anyone else who comes across this: I had the same question because it's not documented in the API reference. You can use a DELETE method. This is what it looks like in cURL:
curl --location --request DELETE 'https://co1.qualtrics.com/API/v3/eventsubscriptions/[YOUR_SUBSCRIPTION_ID_HERE]' \\
--header 'X-API-TOKEN: [YOUR_API_TOKEN_HERE]'