For more on this topic, go to HNsearch.com and look for [patio11 pathological customers].
The nice thing is, they can be ignored, as long as you do it tactfully. If you lose them as a customer, it's no big loss. A single extremely high demanding big customer that pays in the ten to hundred thousands is hard to ignore. In fact, their high demands can run your business into the ground, if you're not careful.
What you do? You shut them out.
Not "remote disable them". But feature/support requests policy has to be simple. Either a user forum, FAQ, etc
You should absolutely not waste your precious time with this kind of work. If they want a refund just give it to them (not sure how easy this is on App Store or others)
Also, if this is so overwhelming, raise the price If they really want this so much they can pay 1.99
You don't need to ignore everything they say, but take it with a huge grain of salt.
Designing say a beautiful UI full of clever widgets is probably a waste of time vs adding features. It just needs to be "good enough" to use, if the users are having problems with it then it is probably more economical to get them in a room together and run a training session at which point they will grumble, stick some postits to their monitors and carry on.
I've developed some programs before that have had bugs and when I've offered to get right on and fix them I have been told by the client "don't bother, I'll just tell the users not to click that button under those circumstances".
With the cheaper app you are looking at ways to do things that will wow 100,000 people rather than just a few.