SpamAssassin can't do that.
SA is a filter that tags messages with information - delivery is the responsibility of the program that calls SA.
If you're using qmail, you can set up SpamAssassin through Qmail-Scanner, which has a patched version able to delete spam at a given threshold. The patched version is here: http://xoomer.virgilio.it/j.toribio/qmail-scanner/
The original version is here: http://qmail-scanner.sourceforge.net Where you can also find a mailing list to ask this question and get a better response than you will ever get from the SA mailing list.