I see this has been listed as a feature suggestion at infopop:
http://community.infopop.net/2/OpenTopic?a=tpc&s=729094322&f=7343077313&m=6283058313 (also part of the "Joe, what would you add?" series of suggestions) but unfortunately I see no official reply to this request. In short, currently the only opportunity to enable email notification of replies to a post is when you actually post. The option isn't listed when editing the post so if you forget to enable it you have to delete the post and re-post. Say you enabled it and the sucker turned into a 10 page posting marathon so now you're sick of receiving notification of hundreds of replies. How to turn it off? Delete the whole thread

Should be able to edit the first post and turn off the notification.
Since Infopop appears to be ignoring this, is anyone up to the task of creating a hack to correct this flaw? If Infopop has addressed this please forward me to their reply as I was unable to locate one. I also came up empty on a search of this site for "edit email notification."