Completely disable SpamAssassin

You can edit the exim startup script, /etc/init.d/exim.

backup your file first, #cp exim exim.bak

then, #nano exim

scroll down and remove the following section

if [ -x "/usr/bin/spamd" ]; then
echo -n "Starting spamd: "
$DAEMONIZE /usr/bin/spamd -d ${local+"--local"} --allowed-ips=${allowedips-127.0.0.1} \
${timeouttcp+"--timeout-tcp="}${timeouttcp} ${timeoutchild+"--timeout-child="}${timeoutchild} \
--pidfile=${pidfile-"/var/run/spamd.pid"} --max-children=${maxchildren=5} \
${maxconnperchild+"--max-conn-per-child="}${maxconnperchild}
echo $RESULT
fi
fi

scroll down a few more lines and remove this section

echo -n "Shutting down spamd: "
killproc spamd
echo $RESULT
rm -f /var/lock/subsys/exim

exit and save, that should do it

If things go south restore backup, #cp exim.bak exim