Is it really important that they not be able to, or is it meant as a reminder to use the mounted directory instead?
What about making your intended directory the root directory of the server (jupyter lab /path/to/mounted/dir? Then the root of JupyterLab, etc. will be the mounted directory and JupyterLab won’t be able to create files outside that directory. That would make it unlikely for folks to accidentally create files in the home directory. They’ll still be able to, permission-wise, but not with normal use of JupyterLab.