This is not currently possible in Cosmo or the server bundle. You could achieve this by placing a reverse proxy in front of Cosmo and disallowing access to the URL for the Cosmo signup page. A future version of Cosmo will allow administrators to turn off signups.
Answer: Because there is a problem with derby on OS X 10.3 (with JDK 1.4) and OS X 10.4 (with JDK 1.5). The solution to this is documented at the Derby FAQ site. Place your
derby.propertiesfile in the top level distribution directory. Note that Apple has issued an update for Tiger's JDK 1.5 that solves this problem.