Locate the User (see how do I find a User if you're unsure how to do this).
Click on their Username to view their profile.
Click on the My Activities tab.
Tick the box next to the Activity you’d like to amend. You can select multiple Activities at once.
From the Action dropdown menu, select Cancel Registration.
In the popup window, you can add a note (which will be visible to the User) as to why you are cancelling them from the activity. Click Confirm to finish the cancellation, or Cancel to go back.