Hello,
Manual deletion of system accounts is *highly* not recommended

If you are stuck half way, then the easiest thing to do is re-add the system account:useradd usernamethen just delete it through DirectAdmin. If you want to go the *other way.. and manually toast the account, you'd need to do the following:
userdel -r username
rm -rf /usr/local/directadmin/data/users/username
#remove the username from /usr/local/directadmin/data/users/resellername/users.list
rm -f /var/spool/mail/username
#remove the users "Include" line from the /etc/httpd/conf/httpd.conf
#for each domain/domain pointer the user has:
rm -rf /etc/virtual/domain.com
rm -rf /var/spool/virtual/domain.com
#remove the domain line from the /etc/named.conf
rm -f /var/named/domain.com.db
#remove the domain from the /etc/virtual/domains and /etc/virtual/domainowners files.I think that's everything.. it's quite a hack job, much easier to recreate the unix account and remove the user through DA.
John