Glad you asked...
To stop birds from building nests you can make your own spikes by using some plywood or two by fours and hammering some nails in them. Set the boards up with the spikes facing up in the area you want to keep the birds away from.
Then again, this answer might help you...
Perhaps you are tired of cleaning up twigs and pieces of mud from your porch, or worse yet bird poop if you want to stop birds from building nests. Fortunately, there are deterrents like decoys of owls and snakes. Birds also don't like building nests by sharp metal objects. So, you can place a board piece with long nails sticking out and they will leave the area alone. For more information look here: Stop Birds from Building Mud Nests http://www.nature.net/forums/load/bird/m...; Prevent Swallows from Nesting in your Eaves http://www.ehow.com/how_1036_prevent-swa...

