Currently in Plesk, there is no way to simply disable or uninstall Spam Assassin. So in order to do so, you musht remove it manually via the shell command line.

To do so, you must first SSH into the server. Once there, you must tell the server to remove the component. You can do so by typing the following;

#rpm -e psa-spamassassin

This is assuming you install the spam assassin component via the plesk updater. If you do not, you will also need to type the following;

#rpm -e spamassassin

It will then go through and remove the spam assassin configurations for each email account on the server, stop the service and then remove spam assassin from your server.