Dissuading wood pigeons from nesting in my barn. HELP!

The reasons I have for wanting to dissuade the little darlings from nesting in my barn are many, various and not trivial. I have knocked down the beginnings of their nest 4 or 5 times a day for nearly a week now and they are still persisting.

Does anyone have any serious suggestions for dissuading them as I am not about to invest in a goshawk?

I know that I should 'live and let live' but there is no shortage of the little sods around here, probably because they're all so well fed on my veg!