Simply: it cannot be done with Torrentflux. It is not a matter od system settings - as all webui users are simply one system user. It is a matter od php code. Torrentflux development has been stopped in mid 2008. In order to have more control over Torrentflux users (  to make i.e. multiuser seedbox ) one should go for Torrentflux-4brt. Much more delevoped. It would be nice to have Torrentflux-4brt installation tutorial here as original Tf is rather ancient and there is no sense to use it.