is it possible to lock a test plan so that no test cases can be added to the plan
One answer
Yes, you can do this.
You could set the state of the Test Plan to "Approved" and no additional material can be added to the Test Plan; this is okay but sometimes setting a Test Plan to "Approved" isn't desirable.
You can also "Lock" the test plan (it's a little icon up at the top by the Cancel and Save buttons, I'm on 7.0.2 so your mileage may vary).
The availability of the functionality will depend on how Roles and Operations are set up in your configuration.