Actually, you can do exactly what you would like for the sales reps.... ie have a Marketo form on a landing page where the form does not prefill and the Marketo cookie does not get set so the rep doesn't become that person. Note, this landing page and form would only used by your internal reps. You'd want a different form/landing page for the website visitor that does all the usual tracking and cookie setting. The forms on the two pages could have the same fields on them, so you can still collect the same data - but they will need to be different forms.
There's another thread with details on how to do it here, although the thread is a little confusing. https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PXGpAAO
To summarize, you would do the following:
1) Create a form with pre-fill turned off for all the fields you want on the form.
2) Create a landing page template where the line that loads the munchkin code is removed. This is the line you would remove: <?php echo $mContext['endElements']; ?>
3) Create a landing page using the above no-tracking landing page template. Set the meta tags to No Index, No Follow so Google doesn't index the page.
4) Add the form to the no-track landing page (add what ever other instructions you want to give your reps on this page as well.)
5) Approve the page and give the reps the URL to this landing page for them to fill in the information.