In short you have to turn the registration plugin off:
If you still keep configuration in the XML file and want to modify it manually you have to make following modifications:
Edit the XML config file and look for a line in the SessionManager configuration which looks like:
<entry key="plugins" type="String">
There are plugins listed which are loaded during the server startup time. You have to find a line with "jabber:iq:register" plugin and remove it. The line you are looking for looks like:
Usually this is the first line on the plugins list. Remove this line and restart the server.