In my email, I go to the top of the Sender column and click the column header to sort in alphabetical order (either A-Z or Z-A). Then all the emails from the same person are together. I click the delete box next to their first email in the string and then scroll down to find the last email from the sender. I use CTRL-Enter in the bottommost delete box and it selects all the emails from the top box I clicked to the bottom most, thereby ridding my email box of every email from the sender.