Try if this works for you, but beware: It's a beta.
Maybe only test with qualities like green or blue for the test!
You'll find the settings in the FCOChangeStuff addon settings -> Crafting -> Improvement quality...
In that dropdown with the quality you need to select the quality which will be blocked. All higher qualities will be blocked as well.
You can enable an exception if you enable the setting to hold the SHIFT key during the improvment. This will allo to improve to higher qualities even if the quality is blocked.