Adobe Photoshop is the best way and you won’t be arrested. Simply begin program and choose a picture of such hero called “rebel scum” and then use your mouse to choose from the available columns. One such colum is called, ‘liquify’. Spend time experimenting and reinstalling image. As it is obvious you might be obsessed with the rebel scum you wish to torture.

If you want to impose the psychological shock of a rebel militant losing his face, you can go one of two ways. The facial skin and muscles can definitely be ripped off by force, resulting in a face that looks like Red Skull from the Captain America movie. I recall someone being attacked by a chimpanzee this way: https://www.youtube.com/watch?v=a3DeJjHAz8I Thus your weapon can be some type of nanobot swarm that quickly cuts all the rebel's facial tendons, muscles, and skin. Or you can shock someone by blanking out all his facial features while still allowing him to eat and breathe. That can be accomplished by applying a thick layer of super strong glue to his face.

Kind of, probably not by heat, but by chemical changes (or something physical or biological that affects chemical structure).

There are collagenases and matrix metalloproteinase (or matrixins), capable of breaking down the collagen that bonds cells of skin and other tissues. Gas gangrena bacteria uses a collagenase (photo) and snake and other lifeforms uses matrixins (photo).

Going more "Doctor Who"-ish one can imagine another technology that makes a similar effect at distance… Something like microwaves resonating to collagen specific frecuencies, or nanobots.

Superheated Steam

If you had a flame thrower like device that fired super heated steam, it could have that sort of effect.

It would cook any exposed flesh and would cause it to peel off the bone

No

A face will burn before it melts.

Different materials react differently to different temperatures. Every material will melt at a high enough temperate. But some materials will undergo autoignition before they melt. A material that auto-ignites must be able to sustain a oxidation chain reaction in air in order to stay on fire. That is, once the material auto-ignites, the oxidation reaction of the material with oxygen in air must be exothermic, so that the reaction causes a follow-on auto-ignition in nearby molecules.

Materials like metals without highly exothermic oxidation reactions will melt into a liquid before the 'burn' in oxygen. But organic materials, generally, will ignite and carry out exothermic reactions. Carbon reacts with oxygen to make carbon dioxide, hydrogen to make water, and nitrogen to make various nitrogen oxides. All these 'burning' reactions will happen at a lower temperature than the melting point of the organic materials that make up the human body, like collagen or keratin or live cells.

For a living creature, the water must be driven off before temperatures will rise enough to auto-ignite, but the fact that creatures are ~75% water means their temperature will stay in the range of 100 C (the boiling point of water) until all the water is driven off. Once the water is driven off, the organics will auto-ignite with increased thermal energy, before they melt.