Now in Plesk, there is no way to simply disable/uninstall SpamAssassin. So in order to do this, you must remove it manually. You can remove the rpm for the spam assassin component.
# rpm -e psa-spamassassin
This is the case inwhich you have installed the spam assassin component via the plesk updater. If it not so, use the following command.
# rpm -e spamassassin
It will remove the spam assassin configurations for each email account on the server, stop the service and then remove spam assassin from your server.