I have read some more exotic ways of culling spam here and kudo's to those people .... but I was looking for a simple solution to rid my mail Inbox of spam (I was getting sick of people offering to enlarge my appendages and make me rich), so here is how I did it with Spamassassin and KMail (I'm using Mandrake 9.1) just running a single POP3 account from my ISP.
Firstly I installed spamassassin ... its on the Mandrake cd's so that was a no brainer. I just made sure that spamassassin was going to fire up on boot ... "chkconfig --add spamassassin" and make sure it was up and running "service spamassassin start".
I then opened KMail ... my email client of choice and made a folder called Spam and then set about configuring the filters.
Open > Settings> Configure Filters in KMail. Click on "new" in the lower left corner don't rename. In the Filter Criteria > select "match all of the following" select "any header" on the first box and "matches regular exp" in the next box and then put a "." in the text box, without the quotes just the . by itself.
Then in Filter Actions choose "pipe through" and in the text box put "spamassassin -a" without quotes and I check the box for "if this filter matches stop processing here" (I have heard that the "if this filter .... " is not necessary but it works for me so I leave it that way).
While still in the Filter Actions click the "More" button and choose "move to folder" "Spam"
The more recent versions no longer alter the subject line (by default) instead they insert a new header line. So you will have to set the second kmail filter to be, filter on 'Any Header' to be "X-Spam-Status: Yes" (without the quotes).
Do a - man spamassassin - on the commandline and it will document all the settings etc that spamassassin is capable of doing. It is quite tweakable if there is such a word.
Thats it ... while it doesn't kill the spam it filters it off into the Spam folder which with Kmail you can right click on the folder and choose to have it dump all the mail contained within every 24 hours.
So far I have had it runing three days and it has caught 100% of the spam - no rubbish in the Inbox ... I am keeping an eye on the Spam folder just in case it grabs something it shouldn't but so far so good.
Enjoy your spam free Inbox with KMail.