Normally you would want to delete the employee (they can later be restored and will have a new login at that time). But if you prefer to temporarily suspend the employee to prevent them from logging in temporarily you can do the following:
On the EMPLOYEE>List page, click the pencil icon 
next to their name
In the Edit Employee window copy their email and paste it into the Comments field and Save
Delete their email (so they cannot retrieve their login).
Then you can click to Reset & print their sign in. This will change their password.
Later if you want to allow them to sign in again, you can add their email back in that field and then click to email them their login instructions.
Medical Leave for 12 weeks. way to suspend