over population of feral pigeons

Can anyone help me with a feral pigeon population that has really got out of hand.  About 8 years ago, my husband and our young son thought it would be great fun to feed 6 white pigeons that visited our garden. Within a short space of time, that number grew to 80!  Racers which have lost their way home have ended up with us, and my husband has actually "adopted" 3 which have taken up residence in my shed.  I stopped him from feeding the feral pigeons specifically, however, I still get around 30 to 40 in my garden each day. I have bought all kinds of feeders designed to deter them but to no avail.  They peck at the seed dropped from the globe feeders, cling on to the guardians of other feeders, sit on the tray of the nyjer seed feeder, and can reach through the ground feeder guardian.  When they are spooked by something, they all fly off at once and scare everything else in the garden.  Spikes on the roof are not an option, and that wouldn't keep them out of the garden.  I have tried frightening them with a kite and helium balloons, but this has not worked.  They all seem to be nesting in a derelict building near to my home, so the numbers will increase yet again.  I don't really want to take drastic action such as poisoning them, but I can't think of any other solution, as we are not specifically feeding these birds.  Any advice or suggestions would be greatly appreciated.

Ann, Haverfordwest