Data Dictionary is an essential system folder for Alfresco, and you should not try to alter the spaces.xml in order to stop creation of that folder.
What you can do is alter permission on that folder, remove the EVERYONE read permission. But then you MUST add that permission manually to each subfolder so that users get permission to read those subfolders, else Alfresco will not function properly. And after an upgrade, review if new subfolders have been bootstrapped in to support new functionality, then you may need to add the EVERYONE read permission again. Note that some folders have extended permissions as well, so be careful what you are doing.
