related lists - update / delete permissions based on form status:

Hi,

How can I restrict users to add / remove / edit Related List items based on my form status?

My Scenario:

I have a Site Information workflow with Field engineers as a related list. We are allowed to insert new field engineers in the New status. Once the workflow reaches final step, My Site Information form has a closed status. At this point, I don't want the users to remove/edit the related list items.

Greatly appreciate your help.

Thank you,

Krishna