On the Web:

If you'd like to stop following someone, click My Profile and select Following. Mouse-over the Following button to change it to Unfollow.

You can also stop following someone by visiting that person's profile and mouse-over Following and change it to Unfollow.

You can block someone from following you by clicking the menu next to the Follow button and selecting Block. Blocking will prevent the listener from following you--they will no longer receive your updates and will be unable to post comments on your profile.
 

On the iPhone or iPod:

If you'd like to stop following someone, tap on Profile and tap the green ‘following’ button next to the person’s name.

You can also stop following someone by visiting that person's profile and tap the green ‘following’ button underneath their username.
 
The button will then become a blue ‘follow’ button, indicating that you are no longer following that person. 

On an Android Device
 
If you'd like to stop following someone, tap on Profile, then ‘following.’
 
Tap the green ‘following’ button next to the person’s name.

You can also stop following someone by visiting that person's profile and tap the green ‘following’ button underneath their username.
 
The button will then become a blue ‘follow’ button, indicating that you are no longer following that person.