If you have admin rights, you can deactivate accounts under your company as needed. Simply, click on your profile at the top-right and choose Settings. Click on Company Settings at the top-center and then the Users section towards the top-left. Then choose the Edit option and click on the trash can next to the account you'd like to deactivate.
*Note: You also have the ability to reset user passwords from here as well. There may be times when you do not want to deactivate the account, but rather change the password to access it. You can also have multiple admins in a company and whoever has admin rights can add/remove this permission to other accounts.