How to assign permission to a list programmatically using SharePoint 2007 object model

How do we assign a permission to a list so that one sharePoint group have 'Full Control'  and rest of the AD/SharePoint users have 'Read' . I have tried using the following lines of code but this doesn't seem to be working.

                        SPList newlyCreatedList = web.Lists[newlyCreatedListGUID];
                        newlyCreatedList.BreakRoleInheritance(false);

                        SPUser oUser = web.SiteUsers["win2008DevB\\Administrator"];

                        // assign Role to the defined User
                        SPRoleDefinitionCollection objWebRoleDefn = web.RoleDefinitions;
                        SPRoleAssignment objRoleAssign = new SPRoleAssignment(oUser);

                        // specify the name of the role definition like [Full Control][Read][Contribute] etc.
                        objRoleAssign.RoleDefinitionBindings.Add(objWebRoleDefn["Read"]);
                        newlyCreatedList.RoleAssignments.Add(objRoleAssign);
                        newlyCreatedList.Update();

As the above code assign read permission to Administrator, but Administrator can still add and delete items.
Any suggestions?