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. You'll see a set of numbers and letters that look like "D0:23:DB:6C:B9:13". You'll need this.
Now, on your Mac, fire up Terminal and type:
(Don't forget to press return.) Take that Wi-Fi Address you just found and very carefully search for it like this:
grep -i D0:23:DB:6C:B9:13 *
Instead of my example, use your Wi-Fi Address. You'll see something like this:
The long string of numbers and letters ending in .plist (not including the colon) is the file you need to remove! (If you don't see anything, check very carefully that you typed the Wi-Fi Address correctly. If you still don't see anything, then your device likely wasn't trusted in the first place. Be very careful to use zero and not the letter O when you type this string!)
How to remove the file? Well, type:
sudo rm xx...xx.plist
where "xx...xx.plist" is the file that you want to remove ("39c1082da11b4920c2298dca702ca3795efbf0cc.plist" in the example). You can use copy and paste for the file name, or type the first few letters of the name and hit tab. You'll have to type your password (and you need to be set up as an administrator on your Mac), but this will nuke the file and untrust the device with that Wi-Fi address.
(My appologies to the CLI-adept at this long explanation, but even highly adept Mac users don't necessarily grok the Unix command line. And, yeah, I know what a MAC address is, but imagine the confusion!)