Q3.9.4: How can I turn off automatic horizontal scrolling in specific modes?

Do (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 do

(set-specifier scrollbar-height 0)

then horizontal scrollbars will not appear in truncated buffers unless the package specifically asked for them.