You should be able to do that from the administration of your project area. In the Web Interface use the "Manage this project area" link to access the administration page of the project area. There you should see a link to "Permissions". On the Permissions page you see that you can change the Project configuration or the Team Configuration. The Plan permissions are on the Team Configuration. Choose the Team Configuration. And on the right you will see which roles have which permissions. Choose the appropiate role, e.g. the Team Member role and under "Planning" section deselect the permission to save the plan for this role. As far as I know, this will keep anybody with the role Team Member from modifying the plan views, but they are still allowed to modify the work items on that plan and save these changes.

hope that helps,

Henning