Like I said before, Warren should do what Christie did in 2011 to get the media to finally believe that he wasn't running: After months of denials of interest, flip-flop and say "OK, I'll think about it. I'll give you an answer next week." Then the following week, make the announcement: "OK, I thought about it. And I'm not running." That was what got everyone to finally believe that he wasn't running, and caused his share price in the betting markets to crash to [near] zero.
The thing with Christie though, was that he literally considered it. He didn't just walk into his mansion, play video games (or something), and keep a low profile for a week before saying "NO!", he actually became genuinely undecided after being disappointed with the republican field and only stayed out because he didn't think he could beat Obama. If he had a reason to think he could beat Obama, he would have run in 2012.
Warren, on the other hand, has not indicated any problem she has with a nominee Clinton, has pretty much come as close as you can to making a Sherman statement ("I will resign immediately if elected") without making one, and has said "No" from the very first time she was asked about it (without ever saying something less certain, or even using the traditional "never say never, but probably no", nope, just plain old "No.").