Open Pulse Audio Volume Control, plug in the USB speakers, and change the ALSA playback to audio controller Analgue Stereo. In VLC, you need Audio > Audio Device > audio controller Analgue Stereo. I assume it's the same for other media software.
On my computer anyway, this now cuts out the internal speakers as soon as the USB speakers are plugged in (I can't use normal jack plug as I dropped the computer on my third day of owning it and broke the jack plug--thank goodness for USB sound cards.)
Mint 17 Qiana 32-bit / Cinnamon