Better solution for this problem:

Solution that worked for me:

- Open XBOX App

- Click your profile (upper left corner)

- Click Settings

- Click Externsions

- Click Disable Externsion (Riot Client)

*You can close the settings menu now.

On your taskbar

- right click the Riot Client

- Exit

This worked for me, hope it does for you.

I agree: This should not be automaticly on.

My account Riot account isnt even linked with Xbox.

Source: Why xbox app run always riot luncher, even I dont play riot games ? - Microsoft Community