Schools where I live are using recorded hawk sounds and have been very succesful in detering all other birds. Here is a web site to check out. http://www.birdbusters.com/scare_bird_deterrent.html
On a humorous note. I heard of a City Library that had a problem with teens hanging out on the library steps playing their punk music on thier boom boxes. They got a lot of complaints from parents and visiter of the library. The library installed speakers and played clasical music during their open hours. Teens left to find another place to hang out.