You can use the RouterDisableMailToGroups setting in your notes.ini file to do this.
Thanks to Declan Lynch for posting these notes.ini variables from the anti-spam session at LSphere'04:
SMTPGreeting=Companyname SMTP server ready at %s (%s is the current date and time) (R5+)
SMTPErrorLimit=# where this is the number of malformed smtp commands before forcing a disconnect. (ND6.03+)
SMTPMaxForRecipients=# where this is the max number of recipients in any message (R5+) WARNING : This discloses the BCC list
SMTPMaxRecipientCount=# where this is the max number of RCPT commands that the smtp server will be accepted per session
RouterDisableMailToGroups=1 will stop internet mail being sent at ANY group in your NAB (ND6+)
SMTPDebugSearchAllDNSBLSites=1 will cause the Blacklist feature to search all sites to help you gather stats on the best DNSBLs (ND6+)