Can I delete the users who are not a part of my organization anymore?

You cannot delete users from your organization. However, you can limit them from viewing or receiving notifications of the activity going on in your organization by blocking them. Go to the ‘Users’ page through ‘Settings’ and next to each user, you will find a drop down menu denoted by the ‘v’ symbol. When you click on that arrow, you will see an option named ‘Block the User’. Clicking on this option will block the user immediately. You can always unblock this user at a later date, if you want to do so.