SMTP Blocking for incoming mail does just that. It'll block the email before delivery if it meets or exceeds the spam weight thresholds dictated in your antispam settings. For example, imagine you have four spam checks enabled for Incoming SMTP Blocking and each of those spam checks have a weight of 10. If the Incoming Weight Threshold is set to 30, that means messages being received via SMTP will be rejected if they fail three or all four of the spam checks.
These settings can be found by logging in as a System Administrator and clicking on Security > Antispam Administration. One thing to keep in mind though, is that because SMTP blocks are done at the IP level and not based on message content, some spam checks do not offer incoming or outgoing SMTP blocking. These resources should give you some more information on this section and how SMTP Blocking works:
Hope this helps!