You may decide that you no longer wish to receive invitations from unfamiliar users. In this case, you can make your Team accessible only through invitation.
To do this, you need to go to your Team's SETTINGS page and click on TEAM PRIVACY tab on the left-hand-side menu. Turn the slide button on by clicking on it once, in order to make your Team accessible by invitation only. This means that only those users who receive direct invitations from you could join your Team.