Just like any other vessel with beer stone....acid. I have used acid #5 from five star with good results. birko has a similar treatment that I can't remember off of the top of my head. If you have an automated keg cleaner(purge rinse caustic rinse sani) cleaner then run as usual the run a second time with the acid instead of caustic. Some brands of cleaning systems have an optional acid cycle where you can line in a vessel of acid and add it in the loop. check with you cleaning machines manufacturer for specifics.