To reinstall I do pretty much the following (ensuring that I delete any left-over SA stuff before I start).
Please check these out for yourself - before using them.
Code:
killall spamd
cd /var/lib/
rm -rf spamassassin
cd /usr/share/spamassassin
rm -rf *
/scripts/perlinstaller --force Mail::SpamAssassin
/etc/init.d/exim restart
tail -f /var/log/maillog|grep spamd
/scripts/restartsrv_spamd --status
Basically - Stop any running spam services, remove SA directories, install SA, restart Exim, check that spamd is behaving in the mail log and ensure that the status of SA reports back proper settings.