Argon: definitely no. If you ignite a pure Argon plasma without a reactant, you will just redistribute the dirt.

Ammonia: this may react with some of the dirt, but I doubt it will convert it to volatile compounds quantitatively.

If you actually evaporated stainless steel onto your reactor walls, you have a wild mixture of metals distributed, so I'd say disassembling and polishing is the best option. But in case of doubt ask Firstnano if they have a description, sometimes manufacturers only provide these upon special request.

Well, sometimes old-fashioned mechanical treatment may be required: Scotch abrasive pads, abrasive flap discs, etc. Followed up e.g. by dry-ice blasting or isopropanol cleaning. A recommended preventive action is to protect the chamber walls by sheet metal liners, metallic foils, etc. (if allowed by chamber design and process).

Martin Heß can also use other abrasive methods like sand blast. Usually the process kit is not expensive, so just have a spare one so the dirty one could be shipped to a cleaning facilities.

Note that the coating affects the performance, not only particles but the entire plasma uniformity etc.

Regarding sandblasting: there are parts to which you can apply it and parts to which you shouldn't. Parts that are far from high-intensity plasma ranges can easily be cleand by that, that's right, but any part that is somehow inside the actual plasma bowl can cause sparks if it gets too rough.

Better to work with experts, there are probably a few at your location.

Any change to the plasma chamber must be considered carefully. Issues might be as straight forward of sealing, to un stable plasma/flows.