Spamassassin itself will not delete spam emails, only "mark" them as spam. If you have qmail-scanner installed, you can set qmail-scanner to delete emails that have a Spamassassin / qmail-scanner combined score. Look at the required_score in /etc/mail/spamassassin/local.cf. Add that to: SA_DELETE="1" # [num], required_hits + sa_delete will be deleted, 0 disables in /etc/qmail-scanner.ini.
If the spam required score is 5.0 and SA_Delete is 1, then all scores 6.0 and over will be deleted. this is for the entire server.