ERQ would be the way to go, currently the application doesn't offer the ability to lock a cost plan, lock selected line items or fiscal periods, or lock attributes. There is a Freeze Date you can set on the entity plan default page but that only prevents manual edit, populate will still change the data for the whole duration of the plan.
Please do submit an Idea and post the link here for us to start promoting it - if one hasn't been submitted yet.
Connie