Apple - How do I un-trust a computer that I have connected my iOS device to?
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:
cd /var/db/lockdown
(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:
39c1082da11b4920c2298dca702ca3795efbf0cc.plist: <string>d0:23:db:6c:b9:13</string>
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!)
With iOS 7
You cannot untrust with iOS 7. Once you respond with "Trust", you cannot change it. If it's not prompting you now when you connect, it means you have chosen to trust it before.
Snippet from the end of this Apple support article titled "iOS: 'Trust This Computer' alert":
Additional Information
Note: If you select Trust, the computer will be trusted indefinitely. If you select Don't Trust, you will be asked if you want to trust that computer each time you connect your device to that computer.
Last Modified: Sep 18, 2013
With iOS 8
With iOS 8, Apple has provided ways to untrust all previously trusted computers (there is no way to untrust computers selectively). Depending on how you do it, this will result in losing your location and privacy settings or losing your network settings or losing everything with the "erase device" option. The 'Trust This Computer' alert on iPhone, iPad, or iPod touch KB article referenced above has been updated as follows:
If you tap Trust, the computer will be trusted until you untrust all computers or you erase the device. If you tap Don't Trust, you'll see this alert each time you connect your device to that computer.
If you're using iOS 8 or later: To untrust all computers, tap Settings > General > Reset > Reset Location & Privacy. This will also reset your location and privacy settings. You can also untrust all computers by tapping Settings > General > Reset > Reset Network Settings. This will also reset your network settings.
Last Modified: Sep 17, 2014
Delete the contents inside the Lockdown Folder on your PC leaving the folder itself in place.
This worked for me, both iTunes and all the iOS devices asked me again for authorization the next time they connected.
you may need to pinpoint the correct file if you want to delete only one authorization.
see http://support.apple.com/kb/TS5335