It depends if the spam is by bot or human, If the latter, then it's not an easy task stopping it but with the new reg user group you can define how many posts need to be approved before the user can post without approval, this way you can act before the content can be seen by others.

To give you an Idea on how to do this read this but do look around for more info.
Code: 
http://www.phpbb.com/community/viewtopic.php?f=46&t=2023775
With posting bots a simple Q & A on the registration page will prevent 98% registering an account.
Code: 
http://www.phpbb.com/kb/article/custom-profile-fields-as-an-anti-spammer-tool/
Regards