Hi NA,
Do all the spam messages have the same sender's address? If so, highlight one of them, then click on Message | Block Sender. If there are various senders, then do it for each. If you make a mistake and accidentally block someone you didn't mean to, then click on Tools | Message Rules | Blocked Senders.. and just unselect the one you need to.
Hope that helps,
Cheers
Liam