I could be extremely wrong, but I'm pretty sure you give them a soak bath, but make sure to either hold them in the water or have a clean rock for them to wrap around so they don't panic! DON'T LET THEIR HEAD GET UNDER WATER they could drown! I hope you get rid of their mites! good luck!
edit: just realized you mentioned this tactic, but also just to let you know, if this snake isn't new and it just got the mites and they havent had them since you got them, make sure you deep clean the terrarium and replace the bedding because something in there might have the mites on them! again, good luck!
WHY AM I SO STUPID THIS IS THE SECOND EDIT YOU ALREADY SAID YOU CLEANED EVERYTHING WHAT IS WRONG WITH ME OH MY GOSH I NEED A NEW BRAIN I'M USELESS LOL