After the Gallic Wars, there existed a commanders order to plunder the cities of the vanquished. What was that Latin command?
ferre et agere is used to mean: to carry off (whatever you can take)
depopulari means directly to "plunder" as does despoliare, diripere, expilare, expugnare, exspoliare as well as a few other words. So using the imperative of any of these would work fine. Though I'd imagine they'd never have the soldiers lined up in formation after battle and say "Ok, now go plunder!"