I think jkerian is right that we can close these as "translation request" questions, but I also think that you're right – we should mention these specifically in the Help Center, just to be as clear as possible.
Right now, the Help Center says:
Direct translations are off topic. While we are very interested in helping people who are having difficulty in their study of Japanese, we are not willing to provide a translation service for cases where a simple dictionary search combined with understanding the basics of Japanese sentence construction would suffice.
Let's give character recognition requests a separate heading:
Character recognition is off topic. In other words, questions which ask "What's this character?" but don't show any research effort will be closed. If you've looked in dictionaries already and still can't figure it out, this sort of question may be acceptable, but please show your research effort and include as much context as you can.
We can still use the existing close reasons as laid out in jkerian's answer, but I think adding this text to the Help Center will make everything a bit clearer for everyone.
If there are no objections, I'll add this text to the help center in a week or two. Otherwise, please leave a comment below or edit the text in this answer if you think it can be improved.