You can actually delete all the files in the folder, but unless you want to reconfigure all your settings, leave backup.rdb and iamdb.rdb files in the folder.The tvDebug.log is probably the biggest, correct? Now might be a good time to post what those logs do.
ZALog*.txt files - you may want to remove older files. ZALog.txt is the current alerts file, the ones with dates in the name are previous ones. Also, you might want to go to Alerts -> Advanced button and make sure it is set to default settings and not logging everything.
xDB*.tmp - these are created when TrueVector (the firewall service itself) is not shut down properly (system crash, using quick shutdown features, etc). If you have a few that's normal, but if there are a large number then you have something not allowing TV to shut down properly. You should be able to remove these files.
IAMDB.RDB - this is your main database file where the settings are stored. This file is NOT generally usable on a different machine. You cannot remove this file.
*.LDB - this is the file that ZA creates from your IAMDB.RDB file, and is specific to your machine. You cannot remove this file.
BACKUP.RDB - The current backup of your settings database file. ZA keeps a backup while running so it can revert back to the last known good backup in case of TV or system crashes or improper shutdowns, etc. You cannot remove this file.
fwdb*.txt - The are only used by ZL in some cases for troubleshooting. If they need them they ask for them when you contact technical support. You cannot remove these files.
tvDebug.log - this is the log for ZA processes monitoring itself. If you have problems, tech support will usually request this file. You cannot remove this file.
**By "cannot" I mean that you cannot remove these files while any part of ZA is running.