If you have recieved enough messages or the accomodation is rented out there are two option to the top right while you're looking at the specifik ad.
You can either “Pause” your ad, it will then be invisible on the portal. But it still is available to un-pause if you need to rent out your accommodation again.
Or you can choose to “Delete” the ad, it will then be fully removed from the portal and if the need to rent it out again arises, you have to create a new ad.