Unfortunately right now, the only way is to undo 1 thing at a time! I am considering adding a way to delete individual items, but am always balancing my desire to have new features vs my desire to have an extremely simple ui. But I will note down that at least 1 person wants this!
yep :) plus I'm planning a big update in the next few days with much more intuitive object/floor deletion controls (and ctrl+z to undo / ctrl+shift+z to redo)