I've tried simple green, pledge, and dish soap with sponges to remove any lasting stuff on the horn but still smell it and can't play the horn too well. What can I use to get rid of it?
Have you tried wiping it down with Isopropyl Alcohol and good paper towels? If so and that still didn't clean it enough.......
MEK (Methyl Ethyl Keytone) available at Home Depot will remove any and all residue. Of course it will knock you out with the smell that's 10 times worse than Flitz and cause cancer but the nasty old Flitz smell will be gone. Do it outside with plenty of ventilation. Read all cautions on the can about wearing gloves and a respirator, etc.
WARNING: Do not even think about MEK on a lacquer horn and don't let it get in contact with the plastic finger button pearls.
I'm being funny about suggesting you use MEK. Having used Flitz though, it's hard to imaging it hangs around after being washed with Dawn Detergent and hot water. All I smell is Dawn Detergent after I wash Fltiz off a horn.
I think something else is going on and you really should air out the case and wash out the horn. Especially if you didn't wash off the horn right after using the Flitz. Good luck.