Username as display name & for comments

Hey guys,

I have the latest version of wordpress with the weaver 2.0 theme.

I am using s2member as my membership plugin, works great so far.

However when s2member creates a new user through registration it makes the default display name the person first name from registration and I would like to make it the username not the first name.
(ie. for comments mainly)
idk if there is a plugin to force comment author name but that would be great too!

I have messed with the code in functions.php of my theme and the s2member files… just no luck.

any help is appreciated!!