Difficult. I know nothing about bee balm, but rust spores generally have trouble germinating at very low temperatures, so depending on how cold it is, putting it outside may slow the development for a while. If you had rust on it last season or if it's a newly acquired plant, then it could be systemic at this stage and unfortunately it'll just pick up again when it starts to warm up. Avoiding fertilising and thinning out the plant will help, as the reproductive spores will have trouble germinating on tougher foliage with better air circulation, but if you really want to be on the safe side, I'd recommend starting over.

You could alternatively try burying the rhizomes, at least 5 cm deep. Generally with Puccinia rusts, which commonly affect plants in the mint family like bee balm, the mycelia are only able to move inside the plant short distances (with P. menthae, it's usually about 5 cm), so anything that grows past that should be free of aeciospores. Although I don't know whether bee balm grows back in the way that mint does.