The guest and the remote login accounts are enabled by default, but there are those who want them disabled, and this could be due to privacy concern (though i noticed that u cant do much in the guest account), and as for the remote one, well, maybe they just dont want to see it there.
Well, to disable them, all you gotta do it fire up a terminal (Ctrl + Alt +T), and paste in the folllowing command:
To Disable both:
sudo /usr/lib/lightdm/lightdm-set-defaults -l false -R false
to disable only the guest one:
sudo /usr/lib/lightdm/lightdm-set-defaults -l false
to disable the remote login:
sudo /usr/lib/lightdm/lightdm-set-defaults -R false
That was it; ought to do the trick.
To Enable, use the same commands, however, make a li’l alteration; i.e. replace all the ‘false’s with true.