It may not be the answer you are looking for, but:
can't disable copilot code reviews
I don't think there is a way to disable this for users that have access to it (even if it is your repo). I had a look, but couldn't find this (although you can disable things like allows agents to run).
Maybe it makes sense to do this, but it is probably not practical, e.g. if I fork your repo, then create a PR (in my fork), should I be able to use my own Github tools, including Copilot review, on my PR (even if it targets a branch in your repo)? That's probably not up to you.
You can configure automatic Copilot reviews, so if the issue is that every time you submit code that Copilot does an automatic review for you, then you can turn that off (by default it is off, so you might have turned it on). I don't think there is a way to disable/remove the button to manually review.
And no way to stop someone else clicking that button; i.e. you can't stop me using a tool that I have access to when reviewing a PR you created (in a public repo).
Is the only option here to migrate away from Github?
No, there are plenty of other options.
If you turn of automatic reviews and never click the button, then it won't review automatically (for you).
As for the button itself, just ignore it.
Even if someone else does run the tool on your PR, you can just ignore what it says. The same as you can ignore if someone manually adds comments (or spam). I don't know if there is a way to block a user (I couldn't see it when I checked some comments on my PRs), although you can Report Content. Maybe there is some other general way to block, which would be a limitation if it can't be used for Github.
But even if you can't block, you can still just ignore any comments.
(Although I suggest reading them, as you may find it has some useful feedback).