You could try prefersEphemeralWebBrowserSession, a new feature in iOS 13, to at least block your web view from sharing session with Safari and be a bit more explicit about making your user login. I'm not sure it speaks to your specific issue, but might be worth exploring.
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
You could try prefersEphemeralWebBrowserSession, a new feature in iOS 13, to at least block your web view from sharing session with Safari and be a bit more explicit about making your user login. I'm not sure it speaks to your specific issue, but might be worth exploring.