The website makes me automatically join random battles while I'm away from the app/website. I did not choose to join, but it does so on its own and it started to annoy me cause of its impact on stats.

you probably mean QM Multi-Day (MD) games. click "Multi-Player" then "Quickmatch" and uncheck the check box

didnt noticed it send my message twice again....

Edited 3/8/2019 21:11:02

https://www.warzone.com/MultiPlayer?Quickmatch=1

As Joi says, uncheck it to not receive the games.

*SOLVED*

Thanks guys,
I am currently on a 19% boot rate due to the issue and it's been solved already.
I will try to join more battles to decrease the ratio.

Well, one way to get a bunch of games done and lower the boot rate is Quick Match. You can even set it to automatically invite you to games. Give it a shot.