Click on the Users tab in the left-hand side menu.
If there are a lot of users registered, use the Search box to locate the user.
Once you have located them in the list or by searching, tick the box next to their name and choose Cancel from the actions drop-down.
Add a note regarding the cancellation if required (this is optional however in larger organisations with multiple administrators, it can be useful to know why a user was cancelled). Click Confirm.
The user will now be cancelled from the activity.