Is it sensible to simply delete all mail that SpamAssassin marks as being spam?

In general, no. While SpamAssassin is very good at picking out a large proportion of spam, it's impossible for a computer to do this job perfectly. You should only delete mail if you (and your users/customers) would find it acceptable to lose mail that might be legitimate. A much better idea is to filter possible spam into a separate folder that can be checked less frequently than the normal mailbox.

But I really really want to do it anyway!

(someone else please write this bit --j.

How can I configure procmail to bounce messages flagged as spam?