As of TeamForge 4.1 SP3, it is possible to disable the creation of new accounts by users so that only a 'site admin' can create new users.
To enable this mode of operations, simply add the following line to /opt/collabnet/teamforge/sourceforge_home/etc/sourceforge_configuration.properties:
sf.disableUserSelfCreation=true
Once this line is in place, restart TeamForge for it to take effect.