Regarding plesk - stay away from gui interfaces for running servers. If you are going to run a *nix email/dns server, then you need to know what is happening on your server at all times. The gui of plesk is great if you just need to get something to work and then want to forget about it, but servers should ALWAYS be run with no gui, just the command line interface. Devoting processor and RAM space to drawing a picture so you don't know what is actually being done and going on puts you right back in the frame that the OS you are probably trying to get away from puts you.
As far as top goes, it usually ranks things by the amount of processor power that they use. You can configure it to show almost anything in any way, and to learn more about it you should use the man command,
As an easier alternative to top, try htop, which still is a command line tool, but it is a much simpler interface. You can find it at http://htop.sourceforge.net
. Most distros have precompiled binaries for htop, but it can be built from the tar.gz file in about 30 seconds. ./configure, make, (root) make install and you're set.
I have built and continue to run/maintain about 5 mail servers. They have loads ranging from about 30 to over 1000 users. Every one of them runs spamassassin. I just checked the machine with over 1000 users, and with dual 1266 Mhz processors and 512 Mb ram, the load has stayed between .2 and .7. The 30 user box (2.5 Ghz celeron, 512 ram) has stayed between 0.02 and 0.09. The first task on each of them was spamd, which is spamassassin. Getting over 2 means something isn't right (unless this is like a 90 Mhz 32 Mb ram box), and getting over 8 is a serious problem. AS far as configuring spamassassin, I would strongly urge you to check out http://qmailrocks.org
, the #1 tutorial on building a *nix mailserver. Even if you don't build a qmail box like the site suggests, they have a great page on how to build spamassassin and clam antivirus.
I have never seen anything about a spam license with spamassassin. Every install I have done or worked with had free spam services. Spamassassin is free, I didn't even know pay versions of it existed. Get rid of the package you installed it from, compile it from source, and you'll be in better shape.
Looks like you're on a redhat/fedora system. I haven't used one of those in a few years, but I would guess spamassassin's start stop command would be something like
/etc/rc.d/init.d/spamassassin stop or /etc/init.d/spamd stop
You will need to know what the name of the executable is on your system. If you don't know, try doing a serch, something like
find / -name spam* -print
That will probably have alot of output, but you can search through and find it. In my machines, the spamassassin configuration files are in /etc/mail/spamassassin/, but I use debian.