Fipomo45
As far as I know that’s not possible. For passengers buying their tickets as a guest (without login), those mails including the QR code are the only evidence/proof of purchase they get, or in other words they’re the actual ticket.
The only workaround is specific to your email account or client: Check if you can set up some rule blocking or moving those specific mails automatically to some junk or trash folder. Just be careful to not delete everything from sbb.ch, otherwise you’d risk missing important mails such as e.g. the half fare card subscription extension invoice.