Hi,
Thanks for the feedback. At present the invoice contact details can be locked using the Store Client Data Snapshot feature implemented in 6.0.
I understand that the desire for further restrictions on changes published invoices. In 6.2 we added the concept of Draft invoices, and building upon this we are considering implementing a new administrator role permission which would prevent users from being able to edit an invoice in a status other than Draft.
Please do let us know your thoughts on that proposal here in the comments or by up-voting this idea.