Press X on your keyboard, then click Delete.

Press any Delete key you have on your keyboard.

Open Blender.
Switch to the "Scripting" workspace.
Create a new text file in the scripting tab.
Copy and paste this script into the text editor.
----------------------------------------------
import bpy

# Select the default cube
bpy.ops.object.select_all(action='DESELECT')
bpy.data.objects['Cube'].select_set(True)

# Delete the selected object(s)
bpy.ops.object.delete()
----------------------------------------------
Click on the "Run Script" button.

Delete blender itself, no blender means no default cube.

Shoot your monitor with a handgun, no monitor means there is no default cube to be shown.

Set your hard drive on fire, this can be done with applying lighter fluid and throwing a match at it.

Become braindead, no brain function means no cube in sight.

The cube should now be deleted. Please let me know of any more methods.