I've run into this a few times. The solution turns out, while simple, is completely non-intuitive.
You've connected a client's phone to pull off all the "tons of absolutely great" videos and photos they've taken to "help" with the project. And you, being the editor kinda-sorta have to use them because heck, the client themselves took them just for you for their project.
You connect their phone (or iPad or whatever) to your Mac via the cable and launch Image Capture and nothing shows up. Both the Mac and the phone just sit there. Wonderful.
Yep. It'll ask if you want to access the connected device. Hit "Trust" on the phone and they'll show up after a few seconds in Image Capture and you can just highlight them all and drag them off and "forget" to use them later on because they're useless and blurry and shaky.
If you want to "untrust" the phone on your computer or they insist they do (Security! they say, "It's the business phone and our IT guys need it to be secure.") what I think you do, is this mess:
Reset the Lockdown Folder.
Yes. Hard as it is to believe that's how it's done.
If you just want to Untrust the phone you just trusted just now, open the Lockdown Folder and then sort by date created and delete the most recent one.