Go to the GM's office. Click on "waiver wire" which will bring up the list of available players on the wire.
Right above the list of names will be some text that includes the words "click here to release a player from your roster." The "click here" is in red will link you to the page where you can release a player without picking someone up.
Note that you can't always do this - for example, you can't release anyone before the season starts. You also can't release a player if doing so would violate any of your league's settings or WIS's roster requirements (eg min PA/IP, minimum number of position players or pitchers on your roster, etc).