By default, Spotlight will index all your mail Mail.app messages and attachments, even those in your Trash and Spam folders. To exclude these folders from the Spotlight results you can add the folders to the “Privacy” section of the “Spotlight” pane in “System Preferences”.

I’m using IMAP on Gmail so your path may be a little different if you’re on another setup. The paths I choose to exclude are

~/Library/Mail/IMAP-user@imap.gmail.com/[Gmail]/Trash.imapmbox<br />
~/Library/Mail/IMAP-user@imap.gmail.com/[Gmail]/Spam.imapmbox<br />
~/Library/Mail/IMAP-user@imap.gmail.com/[Gmail]/All Mail.imapmbox/Attachments

You might not want to use the last one if you tend to keep attachments in the mail message but I usually save the important ones to a real folder.