Methods to track behind cheating on exams using on-line sites?
I can think of a few approaches that might help. I'm not sure pursuing the sites used will be very fruitful. There are always disposable emails they can sign up with, Tor exit nodes they can route traffic through etc. It's a fool's errand.

Oral interviews post-exam

Ask a random sample of students for an interview after they've submitted their answers. With the script in front of both of you, ask them about their thought process when answering the question and evaluate the extent to which they can confidently discuss the subject.

Cons: time-consuming, potential for false positives from students who forget most of the content after taking the exam/have other assessments to study for/don't cope well with the interview pressure.

Issue unique questions per student

If you have control over the online assessment platform, you may be able to issue unique questions per student to catch out blatant copy-pasting of the question text in order to publicly outsource an answer. I am not proposing that a paper is manually put together per student; this seems like something you'd want to automate.

Cons: the ability to do this well will vary from subject to subject. It might be easy to generate lots of linear programming problems of comparable difficulty if your exam e.g. assesses the student's ability to apply the Simplex algorithm. But, explanation questions may be more difficult and lead to a perception that some students unfairly get 'easier' or 'harder' papers.