Sorry but why didn't you write the procedure straight away? They need help. And probably many more people. I took ages to find how do do it because it's hidden. Here are the steps:
1) In the extranet, go to the ‘Inbox’ tab.
2) Select ‘Booking.com messages’.
3) Click on ‘See contact options’.
4) Select the ‘Account’ topic.
5) Select the ‘Terminate contract’ subtopic, and complete the process by answering all questions.