In 2.0.6 and 2.5.0 the UserService deleteUser method is going away. This is due to a transaction issue in the core UserManager#deleteUser method. The UserManager#deleteUser method is also disabled in these versions because of this.
In the web service we opted to remove the method, rather than have the WSDL reflect a method that would only throw an Exception.
The method will once again return in a future version. In the meantime we recommend using the UserService disableUser operation instead as this will prevent the user from contributing any new content.