This post describes how to hide the search control on default.master. The steps described below are applicable for other master pages, as well.
NOTE: To reverse the changes, simply set Visible="true" on the placeholder.
I decided to write this post as we've seen several support cases with questions on how to hide / manipulate the search control on master pages. This post just covers hiding the search box. There are multiple ways to hide the search control, to be sure, including using CSS (ex: "display:none;").