Shkreli's strategy was to be exceptionally creepy. He started by sending Duca a mock invitation to Trump's inauguration, and followed up by changing his profile pictures to include altered images putting him and Duca together. He even registered a marrymelauren.com web domain. Shkreli told The Verge that he didn't believe this was harassment, in part because Duca didn't explicitly tell him to stop, but that's simply a lie. There's no question that she objected, and you don't have to directly insult or threaten someone to harass them -- if this wasn't obvious trolling, it would be considered stalking.
The ban is only temporary, but it shows that Twitter is taking slightly swifter action against harassers. It took the social network ages to ban Milo Yiannopoulos despite him frequently whipping trolls into a frenzy -- this took just a few days after the campaign began. The concern, as usual, is that Twitter's response might have been prompted as much by the status of those involved (and the prospect of bad press) as the actual incident. Duca notably asked Twitter CEO Jack Dorsey about Shkreli's behavior roughly 2 hours before the suspension. Would the response have been so speedy if she weren't a verified user reaching out to management? It's hard to say for sure, but the turnaround would ideally be this quick for every harassment case, not just the high-profile examples.