Document everything.
Talk to your line manager. Copy them into all emails that you exchange with the student (helps build the narrative, and also helps you avoid putting anything unfortunate in the email, since you know you have an objective audience...)
Look at your university's code of practice for behaviour of research students. If the student is definitely contravening that code, then you can talk to your line manager or graduate dean or whoever about putting disciplinary proceedings in motion.
Shift across to a shared supervision arrangement, 50/50, where the other supervisor is also present in all supervisions and reads all work (that also is a really good way to give you an opportunity to re-evaluate the situation - your colleague will be seeing the good in their work and will be seeing them as a valid human being, which will help you to recalibrate).
Remember that there is a huge imbalance of power, and you are sitting on the comfortable end of that particular scale. Your lack of support could cost them their PhD and their (dreamed of) career.
(and yes, I've been in this position from the supervisor's point of view, a couple of times. It's easy for me to sound glib at this end of it, where the PhD is completed and the relationship has been mended by time and hard work on both sides, and the student has gone on to a prestigious postdoc; I entirely appreciate how all consuming it seems during the sleepless nights phase...)