I know this is an old thread but given there was no answer...
No, you as the home owner can't. HOWEVER the hydro company can do it by pulling the meter head out. That will kill power to the main panel. I don't think they will do it willy nilly though, they will probably want a licensed electrician involved and/or an inspector etc. If you're just replacing a breaker you can just turn off the main though as it will de-energize the bus bar in the panel making it safe to work in there, just don't poke too far into the main part as the incoming part is still energized. You only really need to pull the meter if swapping out the panel or doing extensive work that might involve moving or otherwise requiring to completely kill power to it.
In some cases you may also have a separate cut off switch before the panel. I have that in my house which is nice as I'll be able to safely swap out my entire panel when I want without involving the hydro company. I don't think it's very common though.
Heard of people live swapping a panel but it's risky. If you know what you're doing it can be done but I personally would not chance it. It's practically electrical surgery. One slip or bad move and you got yourself a huge arc flash situation, 1,000's of amps. (whatever the upstream transformer can handle) Gonna be a shocking experience. Good news is, unlike industrial stuff, there should be something left to burry at the funeral but it will be closed casket.