Steps to reproduce:

1. Create host with template
2. Template creates items of type External Script
3. Template does LLD on External Script items to create prototype hosts
4. Prototype hosts is created and also contains 1 External Script Item

Result:
Items on prototype hosts cannot be deleted - since they come from Template

Prototype hosts cannot be deleted because they have constraints to Items, With Constraints to Interface, therefore they or the parent/seed host cannot be deleted

Expected:
Some way to delete Parent host and / or Prototype hosts. 

Workaround:

Delete Items on Prototype host from MySQL

Delete Interface on prototype host from MySQL

Delete Prototype host from MySQL