My rented apartment was advertised on bookings.com...

My rented apartment was advertised on bookings.com for those on a holiday trip to spend a few days at my place. A few days later, the landlord saw the post online and demanded that I out down the post, which I did. However, on that same day, the first guest on a holiday arrived at the property. This was reported to the landlord who confronted the guest and refused that we host them for two days. On the same day, the landlord issued us a contract termination letter with no notice period. Apart from that, the landlord is demanding a penalty equivalent to yep months' rent. My question is, how serious is this offence, and is it lawful for the landlord to impose this fine even when it is not stated in the contract?