Troubles uninstalling oh-my-zsh?

just run uninstall_oh_my_zsh
from here https://github.com/robbyrussell/oh-my-zsh#uninstalling-oh-my-zsh

Try

/usr/bin/chsh -s /bin/bash
rm ~/.zshrc
brew uninstall zsh --force

Have you tried just running the commands from uninstall script by hand? It's really straight forward: https://github.com/robbyrussell/oh-my-zsh/blob/master/tools/uninstall.sh. For the most part it just removes OMZ and attempts to restore a back up file:

rm -rf ~/.oh-my-zsh
rm ~/.zshrc
cp ~/.zshrc.pre-oh-my-zsh ~/.zshrc
source ~/.zshrc

In new version, just run uninstall_oh_my_zsh from the command-line.

See https://github.com/robbyrussell/oh-my-zsh#uninstalling-oh-my-zsh