it depends on how it is generating the responsiveness. most likely it is with media queries in the CSS.
there are usually about four or five and each will start with the tag @media –so try deleting different media queries one at a time and play with it to see what happens, then remove the ones you do not want. when you settle on a setting you like, then keep the css code, but delete the @media line (and don’t forget the closing bracket at the end).