This answer states that it is possible to unlink accounts, but it seems to no longer be possible. I was advised to open a bug report, which is what I believe I am doing here. ;)
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That answer is outdated, since we unified the login and account system this is no longer possible. You can now login to all sites with a single set of credentials which work everywhere, but as a result un-linking is no longer supported.
Once we phase out the last of the internal usage of GUIDs for "accounts" (the previous meaning), even the concept (read: PITA if you're a SE developer) of association will go away, as it's now automatic by design.