As they're no use for anything once bolted, why not leave the stems to flower? Bees

absolutely love them, and they won't be getting in the way of the good onions. When your crop is ready it's easy to see which have bolted as the dried flower stems stick up whereas the leaves of the good onions have died down.
If you leave them long enough you get mini onion plants growing round the flower head, and these can be potted up for a free crop.
Some years I've had a good onion crop, but mostly I get a few bolting, or they're tiny and puny. I have had a monster crop once or twice though. This year all is looking good so far

I hope not too many more of your crop bolt - very frustrating after the work of planting them.