commandtoclient(%client, 'setAdminLevel', %client.isSuperAdmin);
and remove the messageAll(*); line underneath it.
setAdminLevel takes either a 0, 1 or 2 for no level, admin or super admin respectively. You also need to call GameConnection::sendPlayerListUpdate() on the person who's level changed to make sure the GUI is updated for all other players on the server. Here is an example of RTB setting a person to super admin:
%victim.isAdmin = 1;
%victim.isSuperAdmin = 1;
%victim.sendPlayerListUpdate();
commandtoclient(%victim,'setAdminLevel',2);
messageAll('MsgAdminForce','\c2%1 has become Super Admin (Manual)',%victim.name);