Many users post questions on several stackexchange sites just to solve some homework problem he/she is stuck at or a single one time doubt and that is all their 'contribution' to the site is. In such cases, how do you identify a one-timer (or anyone who does not consider stackexchange too be a two-way mutual flow of information) and how do we deal with them. Are they supposed to be answered so more of such one timers are encouraged or should we encourage them to increase their participation and contribution towards the site before answering?
Well, we already have a homework policy, which disallows most such questions. Flag a question if you feel that it is homeworky.
We can't easily catch cross posters without rummaging through their profiles, but such things can be flagged too.
Remember though, if a question seems to be useful to a wider community, even if it comes from a one-time visitor, it's allowed