Once inside your book page, the first thing you'll see is a tick box making the book "available". Just uncheck the box and click Save at the bottom of the page. Your book will be removed from sales within 24-48 hours.
Don't delete the metadata before disabling the book. A lack of compulsory metadata will mean stores will reject the update and so your book will not be disabled.
Here is how to remove several books from sale at the same time.