This kind of user is called an ***ask-and-run***.

It is even mentioned on Meta Stack Overflow:
[Dealing with “ask-and-run” questioners](https://meta.stackoverflow.com/questions/256788/dealing-with-ask-and-run-questioners)


----------

**Bonus:** If we follow the same pattern, we can also come up with a specific term ***ask-and-idle*** for users who post a question but stay idle (but don't disappear/leave) without accepting an answer, commenting, replying to people, etc.


----------

There is also the term ***Hit-and-run posting*** (thus *hit-and-run poster*) but it is a more general term and it is usually used for a one-off forum posting posted by a hit-and-run poster that ignites discussion. The motive of a hit-and-run posting is usually *flamebaiting* (posting a provocative or offensive message).

> Hit-and-run posting refers to a tactic where a poster at an Internet forum enters, makes a post, only to disappear immediately after. The term comes from the hit-and-run crime on auto vehicles, in which the driver hits another car or person causing an accident and runs away. It is also known as making a "drive-by" posting, a play on the phrase drive-by shooting. The post often consists of a lengthy text making lots of claims that can be, but are not always, on topic.

> http://en.wikipedia.org/wiki/Hit-and-run_posting