How to restrict/cut power output to a USB client?

...or, how not to charge my phone when I'm using it as a GSM modem in USB tether mode?

I can use it as WiFi tether modem, but it wastes battery both of the netbook it supplies connectivity to, and on the phone side. In USB tether mode I would avoid the power-hungry WiFi connection, but instead the phone wrecks the netbook battery by charging from it. I'd like to be able to maintain the data connection while disabling or restricting the current fed over the power lines; let the phone and the netbook use their own batteries without draining each other.

I'm using Kubuntu 14.10 but any system solution is okay, I'm sure I'd be able to adapt it. Even hardware solutions would be okay - cut the power line in the USB cable or solder a large resistor in maybe? Would that work?