Like messages, activities can't be deleted for safety reasons. But you can close an activity that you created once it's completed or if you no longer need it.
On the web, select the specific activity and go to the more icon (three dots). Then, select the option to close the activity.
Once an activity is closed, participants won't be able to respond, send money, or participate in the group conversation. You can always deposit funds to your bank account, though.