The following is the best way to completely uninstall node + npm:

    go to /usr/local/lib and delete any node and node_modules
    go to /usr/local/include and delete any node and node_modules directory
    if you installed with brew install node, then run brew uninstall node in your terminal

    check your Home directory for any local or lib or include folders, and delete any node or node_modules from there
    go to /usr/local/bin and delete any node executable

You may need to do the additional instructions as well:

    sudo rm /usr/local/bin/npm
    sudo rm /usr/local/share/man/man1/node.1
    sudo rm /usr/local/lib/dtrace/node.d
    sudo rm -rf ~/.npm
    sudo rm -rf ~/.node-gyp
    sudo rm /opt/local/bin/node
    sudo rm /opt/local/include/node
    sudo rm -rf /opt/local/lib/node_modules
    
Source: http://benznext.com/completely-uninstall-node-js-from-mac-os-x/