Hi Tom,
You can mark the form protected which will prevent the edit in designer button from showing on the tool bar. Go to Tools-Form Options-General and click "Enable Protection". In addition, if you don't give them read-write access to the file-share (or web page) where you publish the xsn, they can still create forms but won't be able to get at the form template.
Hope this helps!
Patrick
Patrick Halstead
Project Manager at Qdabra