Sorry there is no real solution other than netting, but if the area is not too large, make a wooden frame that sits on the soil so you can attach the netting to this to avoid the birds getting underneath.
The other alternative is a high power air rifle with a telescopic sight and quite a bit of patience, but it is quite an effective solution in the longer term! Pigeons also make a very pleasant meal if you can be bothered to pluck and clean them!

My method is a little less dramatic than Bertie's. Lol. I actually give the pigeons an alternative food. Corn etc. near the brassicas. It is not very cheap but round here it works. Otherwise I can send you an excellent recipe for pigeon in red wine. :o))

I had a pigeon problem earlier this year and raided the CD collection. Tie the CD to a length of string with the other end tied to a garden cane and the cane pushed into the ground in the brassica bed. I used quite a lot of these and it seems to have worked in detering the pigeons. It is a lot easier than messing around with nets.