https://bmwixforum.com/account/account-details
Just go to that link and on the left bottom is the self-delete for your account. There is a time wait for it to become effective in case you change your mind and log in after you have started it.
or if you are in a hurry, I can do the delete for you.
I loved to take part on the BMW iX forum discussing issues about my BMW iX m60 buth the start was very bad and even cancelling the account takes weeks. It's a shame.