In Mac OS X Server v10.6.2 and later, volume availability should not be affected by network users logging in to the home folder server via SSH. Apple recommends updating to Mac OS X Server v10.6.2 or later if you allow network users to SSH into the home directory server.
Please note that logging in at the Login window of a Home folder server as a network user with shared Home folders is not recommended. See Mac OS X Server: Don't log in to the server with a network user's account for more information.
You should actively control which accounts can use the Remote Login or SSH service on Mac OS X Server. You can control which accounts can use this service with Service Access Control Lists. These can be set in Server Admin. Instead of choosing individual users in Server Admin, you can create a local group with Workgroup Manager called "directaccess". This local group should contain all of the accounts you wish to allow access via SSH, such as server administrators. Be sure to add the local administrator account and the System Administrator (root) account access to the "directaccess" group.
Once the "directaccess" group is created, you can allow that group access to the SSH with the Server Access Control Lists in Server Admin.
For more information about Service Level Security and Service Access Control Lists, please see this webpage.