A couple of options:
First question to you - are you using SSO? We do, and I never provide logins to users, so once they leave the organization, they have no way of loging in even if their accounts are active.
You can also set a rule for deactivating accounts after a certain number of days of inactivity.
And the most manual way of doing this is to get lists of employees who left and manually going through your accounts and deactivating them. Had to do that on an old brand prior to SSO implementation. Painful....