What is the Messenger Service?
The Messenger Service is (supposed to be) used to send messages between computers. It has nothing to do with Windows Messenger or other IM software such as AIM.
Why do I get spam through it?
As mentioned above, it is used to send messages between computers. Spammers can use this service to send you spam.
Is it safe to disable?
Yes, as long as it isn't used on your network for some reason. If you are on a home network, it is not only safe, but recommended to disable it.
How can I disable it?
1. First, go to Start | Run and type services.msc, and click OK
2. Find "Messenger," right-click it, and click Properties
3. Stop the service if it is running
4. Change the Startup Type to "Disabled"
5. Close the Services window