Are they using the send form to oiption in InfoPath? This can be turned off in form options.
If they are taking the initial email and forwarding the email then there would be a couple options.
Just put the attachment or link to the form stored centrally and remove the view from the body.
Also you could use BCC so that they would not see the other recipients.
You can also setup a different reply to address to use via a rule in Outlook.
That would cripple this ability.
Or use a company policy or something and try to get them to comply. :)