Tenant ending contract early, issues paying rent owed

We had a tenant recommended to us by a previous tenant that was moving out. They signed a contract and we exchanged keys etc on May 14th.

2 weeks later on 29th May, it turns out they never moved in and then asked to leave. We’ve been flexible and allowed them to give 2 months notice and went about finding new people.

We have found someone, but the tenant that never moved in is unhappy with their move in date, feels it’s too far in the future (1st July) and doesn’t think he should pay any more rent.

He is now threatening to list the property himself and find new people to move in sooner. I was firm and clear that it is my property and I decide who lives there and his decision to move out/never move in shouldn’t mean that I am out of pocket. I have been reasonable and suggested we split the difference but he is still being awkward. I know I can retain the unpaid rent from the deposit, but am worried he will relist the property and try to find someone else!

Is there any way I can flag the address etc on OpenRent to ensure he doesn’t do this.

I cant recall whether Openrent demands proof of ownership/consent to let, but you could ask them.