How to remove system updates (such as emails) from the activity stream in an Incident Record

So I have a employee who forwarded an email to the Service Desk that created an Incident.  The only problem was that PIP was contained within the email.  So, that email was included in the activity stream

The technician assigned to the ticket gets an email that contains the info

The technician updating the ticket generates an email to the caller (employee) and the technician that it was update and it contains the original email (with PIP info)

Now, they want me to remove all those emails from the activity stream due to the PIP info

I have tried via the history but even though I get prompted everytime I "delete' the emails, they are still there when I return to the incident

So, are there any specific steps to do this action short of deleting the entire record?

Thanks