As trickykid said disabling a service is somewhat distribution dependent but the command
ougth to do it. It will also suffice to rename the file(s) instead of removing them. In that case you want to change the capital S to something else; e.g. a low case s.
Disabling multiple log in is a bit trickier as this is not a normal thing to do; one trick could be to check if the user is already logged in with the help of the `who` command in the users .profile file and then force an exit. Why do you wnat to do it?