Login on ttyS0

I installed mgetty service on a linux Red Hat 7.1 in order to have an access point from external to manage assistance on a couple of unix/linux machines. The service functions properly. Now I would like to allow only an user to login into the linux machine on which runs mgetty when the attempt comes into ttyS0 (/dev/ttyS0=modem for mgetty). All other logins present in the system must be refused on a connection on ttyS0. I looked for /etc/login.access or /etc/usertty files but there aren't. Tried to create a /etc/login.access file but not solved (perhaps I wronged some passages).
Any suggestion?
Thanking You
Andy