I have recently had to dig over a couple of large borders and remove couch grass from most root balls. Everything was just starting into growth. I needed to wait to see where every thing was. I did it with all my Monardas as well as everything else and the rootballs had to be split. It couldn't be avoided. But I re planted them fairly close together and there was no harm done.

I would try some Grass-B-Gon on a small area, to see if the Bee Balm would be damaged. If it isn't (likely outcome) spray the rest of the bed. If it is (unlikely, but possible) put on a dishwashing glove (a "Marigold", to you Brits) put a cheap string glove over that, wet the outer glove in RoundUp concentrate, and CAREFULLY wipe the grass blades with the RoundUp, avoiding the Bee Balm foliage assiduously. You may need to repeat a couple times a year. I would also put in an edging to keep the adjacent grass from creeping back in. The alternative is to dig up the bed--also putting in an edging--sort out the grass, and start over.

Thanks to all. I can't wait to rid my BeeBalm of all that grass. It is one of my all time favorite flowers!