You cannot delete previously added sample columns, since deleting could mean deleting them for other users as well (for the ones you would invite to your team). However, you can hide the columns. Click the Columns button and deselect the column you do not wish to display in a sample table by clicking the eye icon. When an eye is crossed the column is not visible.
Furthermore, subsequently added columns can be deleted. Simply click the Columns button, select the column you want to delete and click the trash bin icon.