In the Fifteen theme, go to Appearance and select Editor. Choose the Search Form (search.php) file under Templates and delete the code within it. Select Update File at the bottom. This will remove the search bar in the main area of your site.
If you want to delete the header search bar as well, choose the searchform-top.php file (again under Templates) and delete the code. Again, hit Update File. This removes the second search bar at the header level.
Always test this in child theme mode first ?? Good luck!