Taken directly from InfoPath help.
1. In design mode, click Form Options on the Tools menu.
2. On the General tab of the Form Options dialog box, under Protection,
select the Enable protection check box.
Selecting this setting prevents users from opening a form in design mode
only when they are filling it out. The setting does not lock the form
completely. Users can still open the form or modify the form's design by
opening the form directly in design mode. However, they receive a message
stating that the form is protected.
You can use the Microsoft Office Resource Kit (ORK) to install policy
templates that disable the InfoPath design mode for all users except those
specified as form designers. To find the latest information about helping to
prevent users from editing your form template, visit the Microsoft Office
Resource Kit Web site.
