To be honest, I think a good computer user should leave evidence behind of what they've done. Deleting log files isn't to be encouraged but that's not the question here.
Have listed a few items below to remove old mail from q and delete the old logs. Please note that these files are almost certainly recoverable so if you are attempting to hide your tracks from anyone with any skill this isn't the way to do that.
Check that the queue is empty.
If it isn't empty then delete the contents of the mailq(this will delete all messages so use with caution)
postsuper -D ALL
Postfix will also log to /var/log/syslog on a debian box by default - I presume it will do the same to a similar file on OSX. Check also that there are no mail.log.1.gz or syslog.0 type files. These are the log files from yesterday compressed to save space.