This article is from the 3b1 computers FAQ, by John B. Bunch with numerous contributions by others.
The easiest way to do this is to edit (with your favorite editor) the /etc/inittab file. Change the line:
ph0:2:respawn:/etc/getty ph0 1200
to:
ph0:2:off:/etc/getty ph0 1200 ^
NOTE: There is a space character in front of the first "ph0" in the lines above. ph0 may be ph1 if you are using line 2 for the on-board modem.
 
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