An air horn works very well. It only needs to be used once with each aggressive dog. they don't forget the sudden noise. My husband has tried it with great success. Wal-Mart carries them in their party favor section. They cost about $4.
Always a beginner...sometimes I even finish.
Every project is a new beginning.
While the earth remaineth, seedtime and harvest, and cold and heat, and summer and winter, and day and night shall not cease