LinkedIn makes it quite easy to delete your account, once you know where to look. Click on “Settings” in the upper-right of the screen once you’re logged into your account, and then select “Close Your Account” under “Personal Information”. You’ll then be prompted for the reason you’re closing your account, and once confirmed, your account will be deleted.
As far as social networking sites go, LinkedIn probably has the most straight-forward account closure process. More details can be found here.