After investigation, we have found that OS X Yosemite somehow doesn't like our digital signature despite it being issued by Apple(!), and as a result thinks the application has been tampered with. There is however absolutely nothing wrong with the app itself.
To run WiFi Guard, simply open
System Preferences, go to
Security & Privacy and set
allow apps downloaded from to
anywhere, as shown in the screen shot below. Now WiFi Guard should run without any problems.