Instead of trying to edit the scripts which won't be of any use untill the new user edits their display prefs why not alter the user table so that the default value of U_ShowSigs is 'no' that way when a new user signs up it's already set to 'no' and they have to edit their display prefs to turn it on?
To do this use this sql command:
ALTER TABLE w3t_Users
CHANGE U_ShowSigs U_ShowSigs CHAR(3) DEFAULT 'no'
That should work.
(as always back your database up before messing with things)