This kind of user is called an ask-and-run.
It is even mentioned on Meta Stack Overflow: 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 asks but stays idle (but doesn'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 user that ignites discussion. Its motive is usually flamebaiting (posting of 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.
