If they have left the company, just redirect their email to who ever is dealing with matters on their behalf, then get a password reset sent to the email ... change the password and let them go dormant.
Really, you want to have any alerts or anything still go to somebody, as sure enough you will miss reassigning something, and why go hunt for something that doesnt exist maybe, or maybe not, you dont know. Let it find you.
Locking them out secures the environment too, and you will probably want any of their input and comments left intact on the board, and not sure how that is handled, so just remap and let it phase out.
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