I joined last night. I must honestly admit I don't know about cancellation. But let's take the example there is a kill switch on your account. Unfortunately they would be storing their personalized key on their side for the operation of their services, and cancelling your account may disable your access to their site, if cancelling IS an option, but I cannot see from outside that they delete their personalized RPC key or not. Again, my position after 30 years in software though, is that is doesn't really matter. If they abused this, so many people would complain to steemit so fast that steemit would kill their RPC keys, and if they don't well, they were never malicious to begin with. Also, if they did, you'd see the activity in your account. They couldn't hide it. It's bad business and there is no valuable reason to "hack" you in the first place. Occam's razor. No reward, no reason to hack. And there is no reward available to them to do so.