A "class extension" is the set of methods that are defined in another
"package" than the class itself. In the system browser with the
"Hierarchy" tab selected, the extensions are shown in the second, top
column.
If you unload a package (Menu option "Remove (Unload)") that extends a
class, the extension (and all its methods) is gone.
Another way to remove an extension is to remove (delete) all methods
extended in a package. In VisualWorks 7.4 I tested doing this after
loading package OracleEXDI. I then deleted the only method
#oracleConversion. The OracleEXDI extension is immediately removed
from the browser. So when all instance and class methods are removed,
the extension is automatically gone.