It is indeed unfair to those 'intermediate' participants when you need to spend time on 'beginners' issues or questions.
On the other hand, it is also impossible to stop those 'beginners' from coming to your training when it is actually for the 'intermediates'.
What you can do is that you have to specify clearly to the class during the beginning of your training that certain fundamental knowledge is needed.
When they start to ask some simple questions, just briefly answer them. And ofcourse if it happens too frequent, you would want to bring to their attention that the class is for the 'intermediates' and suggest that they join the 'beginners' class for better foundation.
In any situation, just remain Professional.
Those pityful 'beginners' also pay for the course, I suppose. :)
Good luck.
5mm.
Singapore.