I have recently bought stadia and realised that my internet is not up to the mark to support stadia, hence I have decided to return it, however i didn't realise that returning the stadia is just not enough, we should also cancel the subscription of stadia separately. Because of the ignorance, I have been charged even when I haven't used stadia subscription after i returned stadia pro.
When I tried to contact support through chat, they say they have clear policy and they already notified. Despite all of that, I have missed to cancel my subscription, hence it looks like I have to pay for the ignorance and not for the service.
Since I didn't get any help from support, I would like to post and check if there is any way to get a refund as i have not used the service, but I have been still charged for this subscription.
you mentioned you have contacted the Stadia support already, but if they don't have the possibility to grant you a refund in that case, then it's probably denied because of Google's policies. I doubt there is anything else you can do here, apart from maybe consulting a lawyer, but I don't know if that's worth the effort here (and if this would be successful is also another question).
Regarding subscriptions, I would always recommend to cancel them anyway if unwanted, because it's not usual that they will get cancelled automatically. At least if you have cancelled now, you will not get any further charges anymore.