Hello Christophe,
To make TestComplete use mapped names for already mapped objects instead of using aliases, just remove the Aliases tree, but leave the Name Mapping tree. If you already have scripts that use aliases, use the [url=
http://www.automatedqa.com/support/viewarticle/12410/]Replace dialog[\url] to replace all "Aliases." entries with "NameMapping.Sys."
Note, that using the Aliases tree is the recommended way to work with the mapped objects. In particular, the Aliases level allows you not to modify the objects names in scripts if the object structure of the tested application has been changed. So, can you describe why you don't want to use the Aliases feature in your test?