If you go to "Administration->Bookings->Item Manager" then....

To delete a single room......

Click on the room in the main list to go to its settings page.

Then, in the top-right area, click on the blue text where it says either "Active Status: Bookable" or "Active Status: Not Bookable"

You'll then get a pop-up box where one of the options is to delete the room fully.

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To delete multiple rooms....

At the top of the page listing all rooms/resources, click on the green Bulk Update button.

If any of the rooms that you want to delete are currently bookable, you'll need to make them non-bookable first (Active Status setting).

The option below it (Delete Items) lists all rooms and resources that are not currently active in the system and they can be cleared from here.