I helped a friend release his stuck #5 POSK damper. Think "strap wrench" and design a tool two people can operate, involving a stick and straps. Working without an improvised tool is a shoulder injury waiting to happen.
The damper assembly can simply fail rather than twist free. We also used some scary solvent like WD40, which we were careful to burn off.
When I had my #7 POSK (before my 23KK), I found a stainless steel nut and bolt matching the damper threads. I'd regularly disassemble the damper, then have a swingers partner-swapping party with these four parts and some food-grade grease. Cleans out the built-up gunk and corrosion. Notice that there are threads that can't be cleaned by use alone, because they're past where the damper is completely shut. A lot of crud builds up past this point; a separate nut can clean past this point.
With this maintenance procedure, there should be no further problems. Although, one gets these problems by not using the POSK. If one isn't using it, one isn't carrying out maintenance either. Give it to someone who'll use it (like I did) and buy a KK.