Stop replying. Keep the messages and the account name, and report the account on the service where the call took place. For adults in the UK, the SWGfL helpline for intimate image abuse handles webcam blackmail and helps with reporting and removing content. It cannot help under-18s or anyone outside the UK.
The helpline answers on 0345 6000 459 from 10:00 to 16:00 on weekdays, excluding bank holidays, and currently works by email only on Fridays. Where the images show someone under 18, the routes set out in the site's 18+ statement apply instead.
Money lost to a scam on a call can be reported to Report Fraud, the police reporting service for England, Wales and Northern Ireland, on 0300 123 2040. The Cyber Helpline gives free, confidential help from cyber security volunteers to people in the UK affected by online harm, sextortion included.