In maya, you can create a reference file without namespace, but not directly with the UI.
If you use the command file to create your reference, use the flag -defaultNamespace, so, the file will be referenced without adding any namespace information on the nodes, but be carefull using this method, clashing name won`t be resolved.
description from the Maya`s documentation:
-defaultNamespace (-dns)
Use the default name space for import and referencing. This is an advanced option. If set, then on import or reference, Maya will attempt to place all nodes from the imported or referenced file directly into the root (default) name space, without invoking any name clash resolution algorithms. If the names of any of the new objects already exist in the root namespace, then errors will result. The user of this flag is responsible for creating a name clash resolution mechanism outside of Maya to avoid such errors. Note: This flag is intended only for use with custom file translators written through the API. Use at your own risk.
Olivier.