How to Change Android Box firmware or stop remote access

[AC Question]

I have a TV box bought with one of the TVIP companies (ATN). They subscription ended and I want to use the box as just Android box with KODI.

They remove remotely any app I download (so I download Kodi in 5 min they will remove it).
My question how to flash the box and make it simple android box without their services or remove all their certificates (option not available) so they can't remove the apps for me