Second the vinegar treatment. We took my son to the dermatologist and he treated them with blister beetle juice. It did kill the ones that were treated, but they just kept coming back. The blisters were painful, too.
We tried over-the-counter freezing treatments, but those were much more painful than the beetle juice.
We then tried treating them with apple cider vinegar and band-aids. We put vinegar on the bandaids and put them on overnight. We'd do this for a week and then let his skin recover for a week, and then treat any new ones that popped up. It eventually got rid of them, with less pain and expense than the dermatologist.