How to prevent users from updating nodes of the hierachy?

Summary:

I need to find a way to prevent certain users to update specific nodes of the hierarchy. Can we use custom validations ?

Content (required):

I have the requirement to prevent certain users to update top nodes within an hirarchy. We initially figured out how to do this by creating dedicated view points - and managing access at view point level. the issue comes when withing the same viewpoint you need to includ an alternate hierarchy. when you do that, the full main hiearchy is also included and can be updated.

questions are:

• can we manage access at "node" level ?