i have experience in removing DA completely.
I have done it few days before. Let me share my uninstall and reinstallation on my RHEL3 experience to you.
first, disable all relevant service like mysqld, apache, directadmin, named, proftpd,etc.
you can refer to the setup.sh script, files.sh to remove the installed items during the DA setup. Personally, I prefer uninstall the RPMs first by using YUM like 'yum remove <application>'. The mysql, proftp can be removed in this way. Then, you can go to delete users/groups like apache & diradmin,etc. and their home paths. Most of the programs are being removed by these ways. But, there are some config files that cannot removed. You can refer to
http://www.directadmin.com/paths.html . Warning: you must do this carefully. Don't delete the important files.
Refer to my experience, I compare my existing server with a clean installed server. If you see a file with httpd,apache like folder, you can removed by using 'rm -rf <folder>'. The most and important point of uninstalling DirectAdmin is remove the DA-installed programs inside the directory /usr . DirectAdmin compile the files will install to this folder. But you must clearly known which files you can uninstall, which files you can't uninstall. The best way is to compare with a clean installed server. I can successfully uninstall and reinstall it without any problem till now. But you must bear the risk of uninstall it manually.