Dan,
I think an excellent discussion (including an example VI) on this topic appears in an earlier thread in the LabVIEW forum. Refer to the following:
http://exchange.ni.com/servlet/ProcessRequest?RHIVEID=101&RNAME=ViewQuestion&HOID=506500000008000000001E0000&ECategory=LabVIEW.LabVIEW+General
If that link fails for some reason, you can find the thread by searching for "can i password protect a control" from the main ni.com/support page.
Regards,
John Lum
National Instruments