Put cats on top of your signage to chase away the pigeons. Then if the cat population on your billboard gets too much to handle then puts dogs to chase the cats....hehehe

Anyway, seriously.... bird spikes would be the best humane solution (instead of poison)....

Or you can raise a few pairs of Peregrine Falcons.... one city (I think in the US) did this to prey on pigeon over-population. Pigeons are the basic prey of the Peregrine Falcon....