We use coyote decoys at work. They do the job but you have to attend to them, move them around every few days and change their head and tail positions or the geese will learn they're not real. I like the automated ideas presented above and the IR sensor waterer would be a favorite, assuming it works on geese. Talk about killing two birds with one stone ... errr ... spray. It repels geese and waters the lawn.
Now me I'd be looking for something more ... ummm .... forceful. Paintball sounds about right but it'll only work when you're there to "man the gates". So what's needed is to replace the airsoft gun on this baby with a paintball setup and I'd be good-to-go.
http://hackaday.com/2009/04/23/auton...us/#more-10543
This might do the trick ("Honey, we're gonna need a bigger tank")
http://hackaday.com/2006/11/17/paintball-minigun/