I’m pretty sure if I accidentally press “OK”, I get charged extra. In addition to the pop-up, I get the same thing as a permanent notification until I reboot – if I try to disable the notification, I get “These notifications can’t be modified”, as it is using the built-in system SIM Toolkit. I am using GrapheneOS. Name and shame: MOVISTAR.
They are absolutely blockable - try disabling com.android.stk and com.android.stk2. Also try looking up if there’s a way to opt out of via via the carrier because there likely is.