1) Welcome to the Party!
2) As you are obviously new here, I have some suggestions to help us all help you:
When you post, please post your entire configuration including (but not limited to) your installation method and vicidial version with build.
This IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "from scratch" you must post your operating system and should also post the .iso version from which you installed your original operating system. If your installation is "Hosted" list the site name of the host.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3) Remember that goautodial is NOT the software you are using. It is the Installer for vicidial. It also has some pretty reports and such, but the underlying software (the owner of the data!) is Vicidial. It can also be installed with Vicibox.com's iso image if you're ever interested (especially when you want to advance to a multi-server installation ... enterprise level!).
4) Off the top of my head ... I'd say you should empty all the tables that end in "_log". Depending on your motives here, you may also want to empty the "vicidial_list" table. Be sure you delete any data in any "custom_XXXXX" tables as well if you're getting rid of vicidial_list data (custom fields data is stored in them). You should also delete all the /var/log/asterisk and var/log/astguiclient files ESPECIALLY anything in /var/log/astguiclient/archive (which is backups which would contain all the stuff you're deleting!). If you have configured FTP backups for the DB or Recordings, consider removing those credentials if the next user should not have access to those files (or the ftp account). Also look at deleting any history in your root linux user and root mysql user (stored in /root/.XXXX ... note the "." in front of the filename, it causes the files to be "hidden", but they can still be easily viewed/edited/deleted and may have sensitive passwords or other information in them). Also look at any files that end in ".txt" or ".log" or ".csv" in the vicidial or agc web folders (No files with txt/log/csv extensions are needed for Vicidial to run, they may be safely deleted). They often have (at least) the names of prior users and (at worst) actual passwords for those users. And delete the "temp file" for uploading leads (listloader_stmts.txt) which is left in the vicidial web folder and contains the last upload of leads. Also delete the contents of the vicidia/agent_reports folder.
Happy Hunting