If you are on DN v8 you will need to navigate to the website you want to manage, then from the left-hand navigation panel select Edit Website > Website Pages. Once you are in the webpage editor, from the left-hand navigation panel select Design > Page Options. Under the Header section of the Page Options panel you will see the Search Box configuration drop-down where you can choose to hide the search bar (see attached screen shot).
The following user(s) said Thank You: ccruz, 2LiveCards
It's a known bug in 7.5 that occurs when you save another option page on the site. For example saving the social site links page would reset the search box setting to default Yes. It's in the queue with our team to get fixed in a future update.