If the messages always come from the same address or combination of addresses you can set up a message rule to delete the messages from the web server without even downloading them to the computer.
Open up one of the messages to read. Click on Actions, select Create Rule then fill in the options and choose what to do with the messages. The one I like is to delete them from the mail server. All this means is that when Outlook scans the email server for incoming messages, any sent from the address you created to rule for will be deleted direct from the web server without downloading them to the inbox first. You can set up one rule to do the same task on several offending addresses.
Another method which is possibly even better is to download and install Mailwasher click here
Run the Mailwasher setup, import the Outlook account and before running outlook to check for messages run Mailwasher. Any unwanted spam may be selectively deleted, there is an option in there to add the sender to a blacklist which will automate the process of getting rid of future messages. The option I prefer is 'bounce' where the email is sent back to the sender as though your email address does not exist. They get a message to that effect, that the email could not be delievered because of porblems with the address. You'll find that if you do this in a relatively short time you will stop getting the spam messages since the sender will be getting miffed at having them all returned undelivered.
Hope one of the above helps.