There are now two "Don't * comments" options for posting. These are "Don't allow comments" and "Don't send email notification of comments".
These assume a user is using the default of "Allow comments" and "Send email notification". These options should either just invert whatever setting the user already has or have sister options which do the opposite. The former has the disadvantage that a later switch of journal-wide settings would invert the one-off settings, which is probably not desireable, but the latter has the disadvantage that it requires client modifications, and it's pretty clear that client updates aren't as quick as they used to be now that there are many different clients and all are maintained by volunteers. (Most clients don't even support the new protocol version yet, which is a pretty vital change to make)
One of these two changes really should be implemented for the sake of symmetry, especially since right now there's no way to only enable email notifications for one specific entry, which would be quite a useful option until the full ESN system is in place.