What can I do to reduce spam that comes through even after enabling the spam box?

If you have enabled the spam box option under SpamAssassin in cPanel, by default anything that SpamAssassin scores at 5 or higher will be diverted to a spam folder, away from your inbox. You can, of course, change the score threshold to lower than 5, or higher, but 5 is generally recommended. Changing this score will change the threshold for an email to be considered spam, for all email.