From what I know of the biz (and, of course, I mean ‘show biz’) actors can file their name as their personal trademark with the Actors’ Guild and no other actor may use that same name (which is why some actors include their middle initial becuase their name was already taken by someone in the Guild). However, if you want to spurn the Guild (and possibly your career for doing so), nothing’s to stop you from using the same name.
Now, Steve Austin of the $6M Man fame is not the actor’s name and it is doubtful that Steve Austin the pro wrestler is in the Actor’s Guild (but, given a few more TV cameos and a movie deal, that might change).
So, we turn to general law. Invented characters (such as Steve Austin, the $6M Man) can not only be trademarked but copyrighted. No one can use that character for whatever reason. And if the name of the character is purposely trademarked (like ‘Superman’), no one can use that name. It’s doubtful that an ordinary name like ‘Steve Austin’ was trademarked (if it even could). The creators of the character would rely on copyright law to prevent rip-offs.
Stone Cold Steve Austin uses the phrase ‘Stone Cold’ to distinguish himself from the $6M Man character… so, it’s OK.