Here is how to do this on a Mac: Go to /var/db/lockdown in Terminal, as Apple instructs in http://support.apple.com/kb/TS5335 . You can remove everything in this folder, which will remove trust from every device -- you can then re-trust those devices you still want to trust. Or you can remove trust for the specific device; this is a little trickier: First, find out the "Wi-Fi Address" of your device. For an iPhone/iPad, go to Settings->General->About, and scroll down.
