This is the paragraph from Bootstrap 3 documentation about disabling responsivness for a navbar:
If using navbars, undo all the navbar collapsing and
Check this: https://github.com/bassjobsen/non-responsive-tb3 and also read https://github.com/twbs/bootstrap/issues/10371.
You can do this by setting the @grid-float-breakpoint to 0 in your variables.less. And recompile bootstrap.
update As mentioned in the comments by @fred-k the above did not make the navbar dropdowns non-responsive. To fix this you will have to set @screen-xs-max in navbar.less to 0 too. This issue will be fixed in future releases, see https://github.com/twbs/bootstrap/pull/10465
Note @screen-xs-max in navbar.less also influence the Navbar forms