How to remove minimum and maximum price completely?

For some of our products we have values for ‘Your minimum price’ and ‘Your maximum price’ which are selected columns on the manage inventory page. - see attached

For some items this is useful, however for this item I wish to remove the minumum and maximum price limits completely - I can’t see a way to just remove the values entirely and have no minimum and maximum price. I have checked on the manage inventory page, and the product page when you click ‘Edit’. Can anybody tell me how to do this?

Thanks