When your students sign up, they have the following explained to them:
This is a University affiliated mentoring service.
Conversations and interactions can be monitored.
Any incidents of Plagiarism, Abuse, or otherwise unwanted behaviour can be seen and will be punished.
As this is explained to the students, and they sign up anyway, we find that there are extremely few cases of misconduct as, basically, they know they won’t get away with it.