It is possible to delete an activity if no registrations have been made. As soon as an activity has been registered to, you will not be able to delete the activity.
To delete an activity, go to Activities>Edit:
Check the activity you want to delete. You can also select an entire category or subcategory.
In the Operations menu, choose Delete activities.
Confirm you want to delete the selected item(s) by clicking Delete.
If an activity has been registered to and you no longer want it to be visible in your store, you can toggle the activity's status to Hidden.