You can always disable the QR code option from the Settings of your slido > Customization > Present mode > Join event via QR code
If you were wanting all of the joining instructions to be removed, this would be an advanced customization that we would apply on the backend for you. These sorts of customizations are available for Professional and higher plans. Please reach out to support@slido.com with your request or additional questions.
However, this will not remove the sidebar space though, it will only remove the text from there.