Remove "Search Everything" box

How can I remove this search box from the sidebar on my site? It appears everywhere and for some reason doesn't stay in the bounds of the sidebar. We don't really have a use for it...

Thanks in advance for any help provided!

UPDATE:

So far, in response to the answer below, I have tried adding the following to the site master page just before the closing head tag:

#SearchBox { display: none; }

<style type="text/css">#SearchBox { display: none; }</style>

I tried to wrap these changes into a content editor web part to see if that will help the CSS to activate. Doesn't appear to work.

I now have the default master page again.