Go to Engagements > All Engagements. Locate the In Progress or Not Started engagement that you want to close. Check the box to the left.
Click on Close engagement from the top navigation
Note: This will cease all email reminders once the engagement is closed and will also remove their access to it. However, if you wish to provide them access to the engagement again follow the same steps but instead click on Reopen engagement from the top navigation. This will also start email reminders again.