It won't completely block designing (InfoPath XSN files are just Xml files
afterall), but you can go to Tools -> Form Options and then "Enable
protection".

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Andrew J. Ma
Software Test Engineer
Microsoft Office InfoPath
http://blogs.msdn.com/ajma
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>I created a InfoPath form saving data against SQL server, is there
> anyway to turn off design mode so user can only download the form and
> fill out the form? Thanks.