Everything on your Board can be deleted. There are two main ways
Any object on the board that has an Edit Panel can be deleted from it. Try double-clicking on the object to see if it has an Edit Panel. If it does there will be a Trash Can icon that you can click to delete the item
If you click-and-hold/touch-and-hold any item on the Board a Trash Can icon will appear in the bottom right of the screen. You can simply drag the item over the Trash Can and release and the item will be deleted
Deleting lots of items quickly
If you wish to delete multiple items quickly, create a temporary Group [How do I create a Group?] around the items that you wish to delete. Then delete the Group by dragging it to the Trash Can icon.
Note. When deleting Groups there is a difference in the two methods. Deleting by dragging to the Trash Can icon will delete the Group and its contents. Deleting via the Groups Edit Panel will only delete the Group and leave its contents on the Board.