Restrict ticket creation to webform

I have a web based template that I want to force users to use to create
new support requests.

I would like to disable the possibility that someone can send an email
to the queue’s address and create a ticket. I would like for them to be
able to reply to tickets via email, once the RT support person has
replied to the initial ticket.

Any ideas on how to configure RT to only accept intial requests from
one script (on the same machine), but allow people to use email during
the later conversations?

Mike Jackson