The first way
Simply tick the checkbox corresponding to the message you want to report as spam, then click 
on the Action bar.
The second way
Drag the message in the right mails list pane and drop it into the Spam folder in the left pane.
The selected email will be moved to the Spam folder. Go into the Spam folder to verify that the selected message has been moved to it.
All messages marked as spam will be moved to the Spam folder.
1. Go to the Spam folder, then tick the checkbox corresponding to the relevant message.
2. Click 
on the Action bar.
The spam message will be removed from the Spam folder. All messages from this sender will be received normally.