Is it possible to restrict users from editing SO Line Item if the Quantity = Invoiced? Should restrict only to specific lines not the whole order.

Do you mean editing in anyway and not removing the line? It shouldn't allow removal of lines with associated records attached.

Yes. Line should stay, just not editable via UI.

By default, it cannot be removed but it can be edited. Our developer had to build a script to disable editing of the LINES.