As far as I know, the Server does NOT have direct control over Clients. When a Server receives a request from a HTTP/S Client, it can send back a HTTP/S response, but the Client decides what to do with that response. The Server could disconnect the Client after receiving the request instead of sending a response back (you can call
Sender.Disconnect from the
OnCheckRequest or
OnDataReceived event), but I doubt this will have the effect you wanted.
Frankly, I find it unlikely that an E-Mail Client will be sending out a HTTP/S request to a Server and checking the response before it decides what to do. It's more likely that the E-Mail Client will be invoking the Web Browser to send the HTTP/S request (and receive the response) immediately after the user clicks on the URL or a button on the HTML page, so ... whatever the Server did after that would make no difference.
Anyway ... here's a website I've found while googling for "HTML inside E-Mail", which you might want to check out:
https://mailchimp.com/help/limitations-of-html-email/