This thread has the command you need:

http://forums.b2evolution.net/viewtopic.php?t=3350&highlight=cron

I use Isaac's antispam update script, which is just a trimmed down version that doesn't require you to be logged in. I also combine it with EdB's recheck hack, so that when I hit the page with cron, it updates the list, then rechecks all of the new entries to clear any spam that I already have. Those hacks have made my life a lot easier.

Here's the recheck hack:

http://wonderwinds.com/hackblog.php/2005/02/07/antispam_recheck_tool_part3

and here's Isaac's:

http://isaacschlueter.com/blog/b2evolution_antispam_recheck_cronjob

As for the cronjob, I use wget to just download the page. I also have it email me the results, but that part is optional.