Bind everyone. Gag anyone that can cast spells or talks too much. Remove any objects they can find, especially on armed people like adventurers. Definitely, remove any spell books or foci. Minimal food and water. Cramped cages or extended periods of long walking while the mercenaries ride in carts or on horses.

I haven't checked the DMG to see if it gives advice on this but here is my mechanical interpretation:
Bound feet reduces speed by half.
Bound hands means no 2H weapons, disadvantage on attacks, and no spells with somantic components.
Gagged means no spells with verbal components.
No foci or spellbook means no spells with material components and cannot prepare spells.
Constitution saves each day to avoid exhaustion. Probably make the save climb a little each day.