One of the most important things you can do to prevent squirrels from overrunning your garden is to make sure you don’t provide them with an all-you-can-eat buffet. Harvest vegetables as soon as they are grown or ripe, pick up fallen fruit, and use shade cloth or black plastic to protect recently planted vegetables and flowers. The black plastic will heat the soil and offer you squirrel protection, although you will have to be careful to make sure your plants get enough water during the growing season.
Since you live in the Greater Toronto Area, many squirrel repellents that work well in dry climates, such as mountain lion urine, cayenne pepper, and mothballs, won’t work very well for you. The problem is that you have enough rain and humidity that any repellents that work on the basis of odor will fail.
What will work for you is an electric fence, with the caveat that you have to have your entire garden surrounded by the fence and it has to be electrified all the time. You will need a double-stranded fence – a single wire won’t work – one wire about 6″ (15 cm) above the ground and the second about 12″ (30 cm) above the ground, the fence kept clear of plant matter, trash, and debris so it current will continue to flow. If that is going to be too much effort, you can try using ultrasonics, click this link to read about the ultrasonic pest control device.
If you want to get a bit more technical, then have a read here about methods, it falls under the heading of integrated pest control management or IPM.