OP - is perhaps a mistake on your part to think that is somehow "your" spot - you have no more rights to be there than anyone else - is what the "public" part of "public lands" might mean.
So as per your dilemma - neither does that other person have any "rights" to that spot - barring guiding or trapline rights, etc., which have been discounted in posts above. Is between you and them in the bush. Has been so for 10's of thousands of years. I think lots of Europe was about sneaking into the King's land to hunt - can read about how that went for them.
Is why I mostly hunt on my own land, or on land that I have permission from the lawful land owner - it works better, usually, unless some parties have traded money - which is illegal, but happens, in Saskatchewan.
By referring to it as "my spot" was intended to imply that it was the spot I was planning to hunt that morning. Obviously I dont consider it MY spot, otherwise I would not have made this post asking people's opinions. That said, a lot of interesting points and perspectives. Here's what we did end up doing... We stayed and set up there anyway. Didnt touch the guy's blind. We left around 10am and he never ended up coming. So had we relocated, we would have given up this spot for nothing and scrambled to find another just as the morning gobbles were about to sound off. We did not have any regrets, I seriously considered leaving a business card to have the guy call me and settle this matter. I wanted to understand what he was thinking and what his intentions were. We're talking about public land within an hour drive of Toronto, and that was a brand new hub blind. Scanned the area after the hunt and can say with almost 100% certainty that there were no trail cams. I'm sure there are many people who would have walked away with that blind. Pretty risky move leaving it on public... probably why it's brand new