In PhD admissions, if a correspondence is opened with a prospective student, is it common for a professor to just stop replying? If a prospective student contacts a professor about his/her work or group and the professor engages (asking for more info, acknowledging the student's existence ;-), etc), is it common for the replies to just stop? I would think courtesy at least calls for the standard easy let down, e.g "Perhaps we should talk after the admissions process," but on the other hand I understand how busy professors are and how spammy prospective students can be.

So I ask those of you "in the know," if you've engaged a student, do you feel inclined to at least let them know you will not be responding anymore? Or do you ultimately just find it easier to not respond (for example if the info you asked for isn't up to snuff)?

(I'm specifically asking about CS/ENG people, but any discipline is free to respond)
As a faculty member, I try to respond to all prospective students but many times the questions are too vague, tautological, or impossible to answer:


Will I get in? Impossible to answer.
Will you work with me if I get in?
Tautological. If you are in my field of study and you get in, it's likely because I want to work with you. But I don't want to say yes because that may be misread as saying that we will accept you. 
What are the strengths and weaknesses of your program? Not easily answerable, self-serving if I deem my own area strong and likely to insult faculty in the areas I deemed week.
Tell me more about your research. Too open ended. My CV is online.


I don't mean to sound harsh but questions that can be easily answered are the most likely to get responses. 

Faculty do welcome e-mails from prospective students, especially those in our fields. When you get 150+ applications, it might be easy to miss someone in your specific area, so a personal message saying that you are applying is always welcome (especially early in the semester --- Sept or Oct -- before things get crazy).  Questions that aren't answerable on our website are also welcome (for example, not all department websites list which students are associated with which faculty; asking about our team members in those cases is welcome). 

If you pique our interest (and we have the time), we may ask for further info from you -- for example, a paper you wrote or other writing sample. While I try to send a simple "thanks" ack message to these responses, I may be asking for something in November that I won't look at seriously until February with the other application material. So don't take the lack or brevity of acknowledgement to mean anything significant.

Note that we may not want to send an e-mail prematurely closing down discussion as we may feel that this is rude to an applicant who may have more questions. So we are in essentially a typical asynchronous communication hang condition.

That being said, I do often wish that I could respond in http status codes:

100 continue
200 ok
303 see other
307 temporary redirect
402 payment required
404 resource not found
406 not acceptable
412 precondition failed
417 expectation failed
503 service unavailable