zsh: stop backward-kill-word on directory delimiter
For recent versions of zsh, you can simply add:
autoload -U select-word-style
select-word-style bash
to your zshrc as described in the zsh manual (also man zshcontrib).
Another option is to set WORDCHARS (non-alphanumeric chars treated as part of a word) to something that doesn't include /.
You can also tweak this if you'd prefer ^w to break on dot, underscore, etc. In ~/.zshrc I have:
WORDCHARS='*?_-.[]~=&;!#$%^(){}<>'