What are some good ways to close an app without using your mouse/trackpad?
If the application is active You can press ⌘ CMD+Q to quit the app. If You want to force quit this app - press  ⌘ CMD+⇧ Shift+⌥ Option+⎋ Esc.

If the app is currently not active hold down ⌘ CMD and navigate to Your app with ⇥ TAB, then without releasing the ⌘ CMD press Q.