Apple - iOS 10 remember to "not trust this computer"

Unfortunately, iOS 10 cannot remember untrusted computers.

From Apple support document HT202778:

If you choose not to trust a computer, you block its access to content on your device. You'll see the trust alert every time that you connect your iOS device to that computer.

There is a way to avoid the dialog every time you connect — blocking the data connection while still allowing the USB power connection. You can do that with either (1) a so-called "USB condom" (SyncStop, Juice-Jack Defender, PortaPow) or (2) a "charge-only" Lightning cable.