You are really using CUPS - the SCO lp command is using CUPS as the backend if CUPS is installed, unless you specifically symlinked lp to the SysV version of lp.
Check the security setup for CUPS.
Also, try creating users.deny as a zero length file.
I don't have access to a 6.0 machine at the moment, but under 5.0.7 it should be in:
/var/spool/lp/admins/lp/printers/<printer name>/users.deny
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