If you have chosen a sitter, you need to click the "Invite sitter to accept" button beneath the messages in the conversation you have had with the sitter. When the sitter accepts, your ad will automatically be unpublished.
If you want to cancel your ad without appointing a sitter, on your 'Dashboard' page, click the 'Unpublish' button for your ad.
When appointing a sitter or unpublishing an ad, always inform the other applicants of what you are doing. You can use the group messaging feature on your Messages page to do this. See the instructions at the top of that page for how to do that.