If you absolutely had to delete the registration item, you would need to go into each registry and remove the item. Once the item has been removed from all the registries, you would be able to delete it. Your other option would be to set the Item Group to Private. To do this:
  1. Go into > onMessage > Approval > Event Registration
  2. Click Edit for the Event Registration Form
  3. Click on Registration Items
  4. Select Edit Group
  5. Click the Radio Button for Private/Secure and mark the box for a Role (suggestion: Platform Manager)
  6. Select Save & Exit
  7. Go back to General Info and Click Save