Next to each number is an "X" icon, and when clicked it allows you to mark a specific number is invalid as opposed to the entire listing. Once clicked, a pop-up will be displayed with several options for reference in the future. This allows you to mark any numbers that are inaccurate or that the prospect no longer wishes to be contacted while leaving the other numbers available.
After clicking "Update Item", a note is left in the listing history for your future use. Numbers can also be removed from the list from the Lead History section.