A spin on the Markus Lankeit's answer, tested on my Android TV.

• Hold Alt key entire time
• Press Tab key (app switcher will appear)
• With arrow keys select an app that is running
• Press Del or Backspace to close the app
• Repeat for other apps
• You can press Esc to quit from the app switcher any time
• Release Alt key

Overall, this task killing keyboard procedure is essentially the same as on Windows 10, for example.

A spin on the Markus Lankeit's answer, tested on my Android TV.

• Hold Alt key entire time
• Press Tab key (app switcher will appear)
• With arrow keys, select an app that is running
• Press Del or Backspace to close the app
• Repeat for other apps
• You can press Esc to quit from the app switcher any time
• Release Alt key

Overall, this task-killing keyboard procedure is essentially the same as on Windows 10, for example.