Currently, we have good methods of sustained, slow dissipation of energy through radiators. They take quite large area (and mass) per watt dissipated though.
I know sublimators were used at least in the Apollo missions, as exhaustible but long-term means of heat dissipation. I'm not sure how efficient they were though.
Still, I wonder what means we have or have plans for, that could be used for rapid dissipation of huge amounts of power in a very short time - possibly in exhaustible, non reusable way. Ablators come to mind, but all I know about ablators is that they are used in the atmosphere, on reentry, and they strongly depend on convection, most of heat dissipated into the air, not into the ablator. I have no clue how one would work in vacuum.
This is related to my question on Harpoon Propulsion - in particular, brakes of the spool. Their energy output would be of orders of a megawatt, over a period of a couple minutes.
They are allowed to burn, melt, evaporate or explode afterwards, but they must keep on braking at constant force while they work. It may be a friction brake, or electrodynamic or any other, applied to linear motion (along the ribbon) or rotary (of the axis).
So, how can we sink that amount of energy?