After you tap Reply All, tap and hold at the end of the list of emails (you may have to tap 'more' to see them all). You'll get a popup menu that allows you to Select, or Select All. Once you've got a selection you can drag the handles to include more or less, just like you do in Notes or another text editor.
Once you've selected a block of addresses choose Cut to remove them from the address field.
There is no way to tap-select multiple addresses individually; but you can tap each address to select it, then delete it using either an external keyboard or the scissor (cut) icon in n the software keyboards tool bar. This two-taps method should be pretty fast once you get the hang of it.
Note: After selecting a single address by tapping on it you'll have that same drag handles you would see if you used the Select or Select All option I first recomended. This would make it easier to select addresses from the middle of the list.