Alright people, after days of trying various methods, I believe I've found the ultimate solution to remove that annoying shield icon. It's not difficult, and it works perfectly for me.
The following procedure removes the shield overlay only from icons and Start Menu items, which means that the shield overlay will not be removed from Control Panel applets and context menus. Here's how to do it, once and for all:
Make sure UAC (User Account Control) is disabled by moving its slider to the lowest position.
Open the Command Prompt. You'll find it somewhere in the Start Menu.
Type the following commands, pressing Enter after each line:
taskkill /IM explorer.exe /F
CD /d %userprofile%\AppData\Local
DEL IconCache.db /a
NOTE: The last command will restart your computer, so make sure you save your work BEFORE executing those 4 lines above!
If you ever experience the shield icon again (I believe that's impossible now), just repeat the third step. Good luck!