How can I remove a linked account without root user permission

Only the root user of the management account can remove a member account from AWS Organizations.

Check which root email address that account is registered with. You should have access to that mailbox; if you don't - check how you can get access. Use the reset password option to regain root access to that account. When this is done, you should be able to login to that account under root credentials, remove any running resources that you don't need, and leave organization. If you can't leave Organization because it prompts you to add a payment method - you can close the account, and it will be terminated and auto-removed form Organization within 90 days.

Do i have a method to avoid being charged from that linked account?