There are no "rules" against it. YikYak isn't the federal government, and it has no such obligation to protect your free speech on its platform. If YikYak wanted to soft-censor -- or even hard-censor -- any mentions of the words "potato," or "HBO," or "Facebook," or "America," or what have you, it could do so.

That said, it's a shady thing to do, and while not illegal, it's certainly generating controversy and bad publicity. Time will tell if users vote with their phones against it. That's the recourse here: not the law per se, but the market.

Now, this would be a different story if YikYak had an overwhelming share of the market, and the DoJ were to view something like this as anticompetitive. But YikYak isn't a putative monopolist in the chat space. At least not at the moment.