Well, as I see it, you have two options

1 (you don't care about past log entries) - Open the log, click clear log and Done => one nice shiny clean log file

2 (you want a daily/weekly/monthly log) - using your computer's file mangler err manager

rename log.txt to whatever you like, eg: logMar2005.txt
I've not tested 2, and the sky could fall in

but I would expect SpamX to notice "OOK! no log file" and start a new one. If it does, just open a blank
plain text document and save it as log.txt
The smarties among you will probably be able to come up with a macro or script that will do this until it's hard coded by Jeff ;-)
Oh what the heck! time to live dangerously

I'm going to test 2 and see what happens

if you see smoke, it's probably just my brain overheating

....
[later] no problem at all, 2 works perfectly as a manual process, I now have a log260305.txt and a new log.txt as fresh as the spam going through it