To clear a column of data, you can use the fill-down function (Ctrl-D). First select the rows of the run table you want to clear data from, and selected the top cell in the column you want to clear. Then delete the data in that top cell.
After that press Ctrl-D or Cmd-D to fill down the empty value, which results in clearing all the data.
So why not just use the "Delete" key? In short, it's because the delete command applies to the text inside the selected cell, not to the selected column.