I've had to deal with users who sometimes exhibit some of the qualities of a help vampire on chat, some of which are unique to Stack Exchange.
They barge in and ask questions. Sometimes they randomly ping people demanding answers. Chat's a place they figure they can get answers right now.
Sometimes it's folks who are worried their question won't be well received - but are not really willing to ask even when told "it'll be fine", and sometimes are focused on the reputation hit.
They're often disruptive, smothering current conversation like kudzu, and sometimes resulting in a certain level of unhappiness or distaste.
While some folks are regulars others pretty much pop in, start asking questions and don't seem to be aware of any other conversation going on. They sometimes even badger folks when other things are happening. It's a bit difficult to hold a conversation for many due to this.
I suppose it's a well known problem, discussed a lot on many metas. There's a 'classic' work on help vampires, which is often referenced. It has serious issues that kind of make it a little less useful as something I'd link seriously.
I'm not really a fan since the way that it's written seems to be a work of humour. It's pretty darned funny, slightly sarcastic, and... not very useful. There's a few other issues with it that are irrelevant in this context but on the whole, there's a lot of it that doesn't lend itself to sober serious intervention.
I've always felt that moderation is about the actions and behaviours rather than the person. A label like "Help Vampire" is easy. It demonises the person, is a pejorative, and gives us no details about the problem.
I've tried many approaches so far - letting people know the behaviour in question isn't fine - and that they should ask the questions in the right place (which sometimes works). Sometimes we get the reply "But I'll get downvoted". Occasionally we even point out it's disruptive.
I primarily talk about chat here - there are very robust ways to deal with folks with similar behaviour on the main site, and the main site is for questions, but how do we deal with folks who absolutely need an answer right now and are going to get a little in your face about it, or feel they have a right to your time and energy?
More importantly - How do we talk to someone we feel has these problems and what would be helpful, serious references when talking about these issues?