I've said it before to people in game but implementing a "pvp log and you're banned" policy is incredibly harsh, and has the following issues:
1 - Minecraft's GUI means the disconnect button is incredibly close to the back to world button on the type of screen that you want to dismiss quickly, being 20 pixels off with a mouse click away from a ban at all times is a rather grim prospect.
2 - This cannot be considered a "standard" rule of minecraft that something like "no hacking" or "no griefing" can be, which means we'll be seeing a whole bunch of people banned their first time on the server (before you bring it up, yes we could put up a sign but the kind of people who skip them are probably a majority of the pvp loggers)
3 - Assuming we're kicking this one log = ban idea in the head and analysing something more reasonable at what point do we draw the line and where will this be kept track of? Is it 3 pvp logs for a ban? For example, imagine you make the previously mentioned mouse clicking mistake three times over three months and are a regular and longtime member, surely you're going to feel more than a bit aggrieved with a ban?
4 - Its been mentioned before but who is keeping track of this seeing as it basically requires an eye-witness present. You mention in your post that it could be policed by users but that would effectively require a recording running at a sufficiently high resolution for long periods of time. This means that each time you're a victim of combat logging you'll be heading to your videos folder, finding the correct file, editing it, exporting it (for the sake of file size) uploading it and then posting a report every single time. You'll probably feel this is something you'd be willing to do but remember with constant new users you'll be keeping this up for the foreseeable future whilst other users who don't have the pc spec or ability to do this will effectively be on their own.
To conclude, I feel this thread smacks a little of you being a victim of combat logging and heading straight to the forums to post this in the heat of the moment whereas sometimes it's best to take a step back and review the options. I'm not saying that combat logging isn't annoying / a problem, the key is finding a solution that's reasonable and appropriate. Ideally wouldn't this be something that's preventative? (quick heads up: the logout "ghost player stays for x seconds" plugin doesn't work with our inventory system) rather than simply relying on reports and bans.
Btw if there's anything you'd like to discuss/take issue with in my post, feel free to drop it here or talk with me in-game/vent.