While I haven't posted on Kboards since 2015, I removed all relevant material long ago (covers, links etc.). For the most part I wouldn't worry too much unless they actually infringe on your rights. Remove all information that you can, but even if there is a way to have them delete your account entirely, they can slow walk it. While I'm not a lawyer, I would be willing to bet there is a way they can hold on to your data. There's likely a legal loophole somewhere. If not, they know they can simply out lawyer most people, burying them in legal fees. For most indies, it's simply not worth it. That's not meant as an insult. I couldn't do a damn thing about it on my own either. I'd need to bring Audible, Podium or Tor in to help me. I'd run out of money a long time before they did.
You see, these companies are shrewd. They're aware of the upheaval they're likely to cause. But they simply don't care. And for good reason. They know that in time the dust will settle and most people will continue doing what they always do - what's comfortable. Even losing a goodly portion of Kboards users is only a temporary hiccup. More will come. Others will stay away for a time then return. In the end, they know that they'll win. In fact, given that the Hugh Howey's and Michael J. Sullivan's of the world posted in the early days of indie, and have not deleted their accounts (likely because they're unaware of the issue) they've scored big out of the gate.