The current functionality in v7.2.6 OOB database (where CI Business objects happen to be set to Soft Deletion, but this is by-the-by) is as follows:
1) You can only delete a CI in Config Managment if the CI is not referenced by an IPC. (Incident, Problem, Change)
2) If the CI is referenced by an IPC, then you must first select the action to 'Detach Configuration Item' on the IPC record before you can then delete the CI in Config Management."
If the CI is referenced by an IPC and you do try to delete it, you will receive the attached error message.