Several people on the Duck Forum have used the clear iodine method.
Soak the foot to soften the skin
Dab 2 or 3 drops of clear (that's clear, not brown) iodine on the bumble
Wait 5 or 6 days (do not apply more iodine, just wait)
If it goes the way it has done for those who use this method, a scab forms on the bumble if there is not already one there
Soak the foot again to soften the scab
Gently work at the edges of the scab to remove it
If it doesn't come off easily, don't force it - dab more clear iodine, and wait a few more days.
I think @Miss Lydia did this with one of her chickens a while back.