special situation

what if this situation happens, can the black king capture white's Queen (since white's bishop cannot move), or not?

I think this is checkmate, but I want to be sure

It's checkmate.

If the king is in check and has no legal moves out of check, it's checkmate.  Capturing the queen would put the king into check.

A soldier won't fight if there is no king to pay him. It's checkmate !

Yes, because when a king dies the economy collapses... of course.

Anyway, the black king can't move to c7 for the same reason you think the white bishop can't move from g3.  You can't apply one without the other.  As I see it that's the main problem with it.

This is like the third time this question is asked in the forum. Why continue to ask stupid questions?

Naa, more like the 100th time.

Yeah, you won't believe how many threads there are on the infamous "insufficient material" rule.

http://www.chess.com/forum/search?keyword=insufficient+material