Not sure if this is the solution
Code:
# These are the standard console login getties in multiuser mode:
c1:12345:respawn:/sbin/agetty 38400 tty1 linux
c2:12345:respawn:/sbin/agetty 38400 tty2 linux
c3:12345:respawn:/sbin/agetty 38400 tty3 linux
c4:12345:respawn:/sbin/agetty 38400 tty4 linux
c5:12345:respawn:/sbin/agetty 38400 tty5 linux
c6:12345:respawn:/sbin/agetty 38400 tty6 linux
The above lines define which consoles are available at a certain runlevel. Taking one of the numbers out (let's say 5) in the bold part makes console 1 unavailable in runlevel 5.
It might be that pressing <ctrl><alt><F1> will still work but that you only get a black screen; I did not test it.
PS taken from /etc/inittab on a slackware box.