Have you used any of your post-flowering harvested lemon balm? Have you liked it? If so, continue. Some folks are particular about the taste and prefer it before flowering. If you are happy, keep harvesting as long as the plant holds up/you don't have frosts in your area. The plant will get pretty ratty as the frosts come closer and so you will probably stop harvesting well before the frosts end your season.

I harvest all summer from my lemon balm plants. I generally save the younger, choice leaves for tea and the older not so tasty ones for herbal tinctures. Regular clipping will keep new leaves sprouting.

FataMorgana

Have you tried freezing your lemon balm leaves in ice cubes (or juice cubes) - one or two leaves per cube. This makes an excellent addition to a fruity summer punch or refresher, quite attractive floating around until they melt.

Can I freeze lemon bam fresh ?

Try a sample. I think you'll find it will go black unless frozen in ice cubes as recommended above.