Normally, I tell them that I don't answer one-on-one questions that can't be answered quickly or require a lot of details but I do try and give them some guidance or a simple answer. I can't remember anyone ever getting mad over it.
I have considered having pre-written emails ready for the typical questions and then promote something with my reply. It may end up causing more problems though because they will think you are providing one-on-one help.
Now tell me...
How do YOU handle questions for help?