(setq truncate-lines t) in the mode-hooks for any modes
in which you want lines truncated.
More precisely: If
truncate-lines is nil, horizontal scrollbars
will never appear. Otherwise, they will appear only if the value of
scrollbar-height for that buffer/window/etc. is non-zero. If you
(set-specifier scrollbar-height 0)
then horizontal scrollbars will not appear in truncated buffers unless the package specifically asked for them.