Run the following commands to remove mongodb from the launch/startup and to uninstall it using Homebrew:
# See if mongo is in the launch/startup list
launchctl list | grep mongo
# Remove mongodb from the launch/startup
launchctl remove homebrew.mxcl.mongodb
# Kill the mongod process just in case it's running
pkill -f mongod
# Now you can safely remove mongodb using Homebrew
brew uninstall mongodb
Just double-check in /usr/local/bin/ to make sure that the mongodb commands are removed.