I've got an anti camp plugin running from Roger Devil (see:
http://forums.alliedmods.net/showthread.php?t=39695 ) which is really good. It's not desigend for the sourcemod engine, it's a seperate dll and dosn't focus exclusivly on anti-camping, although that aspect of it is quite mature now. I run it on a 20 user server with no noticable ill effects on play, although a source mod version may be more efficient for larger servers, I have to admit ignorance as to the pros and cons of the two methods.
Rather than trying to put you off though I thought you could use the maturity of this plugin to shave a little time of working out features as a number of the likely exceptions have already been included into this.
For example I run it on my Gun Game server where I have the unusual rule of you may only camp if you are on a sniper rifle and this lets me add immunity for those weapons.
If you dont' have a sniper and meet the camping criteria then you get beaconed and every 2 seconds loose some health. In what I think is an odd bug rather than a design feature (but it happens to suit me) when a campers health is less than that which would be slapped of them their health won't drop any more and the console will be filled will alerts that the kill command couldn't be executed, they are however still beaconed until they stop camping or get killed.
Oddly (as I'm usually all for slaying people as a means of behaviour correction) I dont' happen to think campers should recieve a slay as a matter of course as you do get some situations where people get pinned and if you slay them for that they probably won't come back but a bit of a slaping they will usually put up with. I know you could try and code something in for if they are taking or returning fire but I'm quite sure someone would find a way to exploit that and it's more load on the server. What would be nice though was if they were slapped to within an inch of their life for two sucessive rounds that they get slayed in the second round as even with this I end up having to slay the most habitual of campers.
In responce to Moggiex's question 5 though, for the most part it will when they see their health fall away or realise their location has been compramised. However for the most habitual of campers you will I'm afraid require a slay to effect any sort of change in behaviour, but even then they are more likely to just go elsewhere, but atleast you're shot of them.
Hope my comments help and if you want to put any quesions to me feel free.