How do I stop a former employee from logging in to my store admin?

You have two options on Settings → Roles & Permissions → Manage Staffs.

Disable the account (keeps their details):

1. Click Edit on their row.
2. Set Status to Disabled.
3. Click Save changes.

They can no longer log in. Set Status back to Active to restore access later.

Remove them from the store:

1. Click Remove on their row.
2. Confirm with Remove.

If they knew any shared passwords, such as for payment or courier accounts, change those too.