Ransoms only work because people pay them, if people stopped paying them then people would stop taking things ransom. Also we were talking about ransomware, not people kidnapping other.
In regards to kidnapping, it is already illegal to pay a ransom under anti-terrorism laws.
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/540539/CTS_Bill_-_Factsheet_9_-_Kidnap_and_Ransom.pdf
