It is possible to accomplish your end goal in this situation, but it requires some thinking outside the box. Since the ability to move documents is an assigned feature right we cannot limit that movement to a specific folder. If I have the ability to move a document from folder A to folder B, then I also have the ability to move a document from B to A.

That being said, I do think that there is a good way to handle this request. With the help of Workflow, I think we can improve this process. Instead of having users move documents around, you can have Workflow do it for you. This workflow could be triggered based on a field value, document tag, or even by kicking off a business process from the client. Users would control when and where a document needs to go by way of one of these three options and workflow would do the actual moving of the document. The end result would be an environment where users move a document into a folder by assigning a tag to that document for example, and they are unable to move the document out of that folder.