2. Stretch a net between them.
3. Catch the passing freight quadcopter.
4. Get the package, get the parts.
5. Profit.
But now the other bandits are doing it too. Add automated defense mechanisms. Intercept competitors. Degenerates into fully unmanned airborne war.
Full unmanned civilian aviation warfare sounds pretty cool.
The vast majority of RC planes, cars, multirotors, etc. work with a 2.4Ghz transmitter (TX) and receiver (RX). In the old days you'd claim a frequency, put the corresponding crystal in your TX, and hope that nobody else would use that frequency. Nowadays we have Direct Sequence Spread Spectrum (DSSS) or Frequency Hopping Spread Spectrum (FHSS) to ensure that you're not interfering with someone else's TX/RC. These signals are a lot more difficult to hijack.
GPS Spoofing is probably a better way to hack into UAV's. The Iranians caotured a Sentinel UAV in 2011 with this method: http://en.wikipedia.org/wiki/Iran%E2%80%93U.S._RQ-170_incide...
I assume taking control of the machine with out permission would not be easy, but possible. Also a game show where the copters fight to the death would fill the void of "Robot Wars" being of TV.