Not everyone will want to be notified every time someone replies to their post ... so ...
By some crazy intent -- it is not possible for admin to unselect this for all users!
(it is set as default - although you can manually untick it each post - it is better to disable it in your control panel if you don't want it)
To disable 'Notify me' in your posts :
Go to 'User control panel' (top left)
Overview
Board preferences
Edit Global Settings
Edit Posting Defaults
Notify me upon replies by default: -- select 'No'