Hey @KevinTheJedi & @ntozier, thank you so much for your help! I managed to achieve what I was looking for.
@KevinTheJedi, the %{user} variable works like a charm, and thanks for the note about the Reject Ticket action, I removed it.
For anyone who is interested in a similar setup, I did the following for the filter:
- Target Channel: the email address for which I only want this to work for
- Filter rule: User / Email Address - Contains - @
- Filter Actions
---> Send an Email (with recipients: %{user})
---> Set Request Status: Closed
---> Disable autoresponse
And boom! It works, anyone trying to create a new ticket by sending an email to that address will receive a message asking them to open tickets via the webform, no ticket-creation-notification will be sent, and behind the scenes the ticket was automatically closed.