I once had a disruptive student who asks for clarification of what I had just said every five or ten minutes. I was so annoyed by her, but then realized she probably has some psychological issues. If I said, "turn in your assignment tomorrow." She'd immediately raise her hand and say, "When do we turn in our assignment?" If I said, "The wall is blue." She'd say, "What color is it?" The other students even got frustrated with her and started to yell at her, "She just said....!!!!!"
I have a similar student this semester. She is actually quite bright and does moderately well on exams (B+/A- range), but she can't see the forest through the trees to save her life. If there were a sign in front of her that said "Mountainguy National Forest," she'd ask me "Professor Mountainguy, why are there so many trees here?" More to the point, she spent twenty minutes in office hours this afternoon quizzing me on inane details from course lectures, like "what's the difference between an example and a detail?" and "I don't understand what [XYZ] theory is." I replied that [XYZ] theory was discussed in an article we read, and she said, "I read the article and I didn't understand it." I asked her what part of it she didn't understand and she replied "all of it." I told her to come back tomorrow with examples of X, Y, and Z components and we'd take it from there. She replied, "what's an example again?"
I suppose psychological issues are possible, but I think it may have more to do with a lack of academic maturity. The student is a sophomore, but very young--no older than 18, I'd say.
Back to the original point of the thread . . . like other posters have suggested, don't gratify frequent e-mailers with immediate responses. Also, if a student really does seem to have a problem with sending unprofessional e-mails, I will gently but firmly counsel them that their e-mails are inappropriate and reflect poorly on them as students. Most of them stop after that.