Before 1967 by pledging allegiance to Canada (i.e. by becoming a citizen of another country). Note - that from Canada's perspective she was still a dual citizen, ONLY from the US perspective pre-1967 had she waived her citizenship.
No - but being on Canada's electoral roll does.
That's all changed now - a natural born US citizen just as Ted Cruz's mother would have a lot harder time giving up her citizenship now.
A sign of the times I suppose, and I haven't taken in to account any special treaties that may have existed specifically between Canada and the US (if there were - there may be exceptions).