Not currently##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