Please allow me some time. I have to discuss with our developer team about this. They will give me the right direction to remove this option. Please keep patience and i will be back to you soon with the right path.
To remove become a seller option from your registration form you have to make a child theme. And paste this code remove_action( 'register_form', 'dokan_seller_reg_form_fields' ); in your child theme's function.php file and add this code in the same place to remove become a seller from account upgrade pageremove_action( 'woocommerce_after_my_account', 'dokan_account_migration_button' );