Google Chrome and other Chromium-based browsers: type chrome://extensions in the address bar. Press the thrash can icon next to the MixiDJ V8 Toolbar.
Firefox: press Shift+Ctrl+A (or open Tools → Add-ons). Press Disable next to the MixiDJ V8 Toolbar. If you can't find the toolbar in the Extensions tab, switch to the Plugins tab and look there.
Opera 12.x and older: press Ctrl+Shift+E (or press the Opera button → Extensions → Manage extensions) to open the Extensions menu. Press Uninstall next to the MixiDJ V8 Toolbar.
Internet Explorer: open Tools → Manage Add-ons. Select the MixiDJ V8 Toolbar and press Disable.