I don't think there's an option to remove this and the admins would have to modify the actual code to uncheck it.
I made it work by using userscripts which requires a new addon/extension on your browser.
This will not work on mobile unless your mobile browser supports userscripts.
Firefox Instructions- Install the Greasemonkey addon
- Install the script by clicking here and you will see a popup with a countdown timer, click INSTALL after it's done. (Screenshot)
- Click here to open a blank pm and verify that the READ RECEIPT checkbox is unchecked.
Chrome Instructions- Install the Tampermonkey extension
- Install the script by clicking here and you will see a new tab open up. Click INSTALL and close that tab. (Screenshot)
- Click here to open a blank pm and verify that the READ RECEIPT checkbox is unchecked.
I can not make it any easier than that.
Let me whether it works for you or now.