Although you could create another class of users, I’d just do the following:
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Change their password to something really strong and random
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Change their email address to an invalid value (or maybe a variation of yours) so they can’t do a password reset.
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Maybe change their description/info to show that they are a ‘past employee’ or whatever generic/polite term you want to use
That way, their articles will still be visible and attributed to them, but the person will no longer be able to add, since you have changed their password.
And, if they have any other access to your site (FTP, hosting, remote access, network access, etc), deleting those accounts is important. Act as if the person would do damage to your site in any way, and block those actions, even if they are leaving on good terms. Disgruntled and ex-employees can have nefarious intents….and better to be safe than sorry.
And I would do all of this sooner than later. No need to give them the opportunity to be nefarious.