You tell o2 what phone number is to be barred (ex partners sim) and request they put a bothway bar on it so he cant receive or make any calls. If they put an IMEI bar on too at same time that will also bar the phone hes currently using which would probably be a good way to *bleep* him off more but just ask for the bothway for the sim.
You then request to IMEI bar the iphone X handset so you may need the IMEI number of that phone but if you dont have it the adviser can check a system called ABS which they can use to track every phone used with his sim. Assuming he hasnt used any more than one iphone X they would bar the most recent iphone X he has been using. They can also find the iphone X IMEI in your orders on their synergy/carex system.
Once the sim has been barred you can request they send you a replacement sim for the number he uses so you could give it to someone else to use. You could even ask them to change the number too once everything done.
Thing to remember is the contract is in your name and the iphone x was your property so it can be classed as stolen if hes sold it. You are in control as your legally liable for it regardless of who it was intended for.
Also to confirm all actions would be taken on his phone number so will have no effect on your iphone 11 at all. The phones are not linked to the original contract if you have swapped them round they are linked to whatever sim is in the phone at the time.
