These "customers" who think so little of your time do not own their own business, nor have they ever worked on commission.
Now you are in a position to make a decision. You can continue to be sweet and helpful, and they will continue to abuse you - indeed, they will take you for all they can, more and more until you finally get fed up and say something less than kind - and then they'll go somewhere else to find a new sucker and badmouth you for being mean.
Normally we get rewarded for being helpful and informative and kind, but occasionally you run into "takers" like these folks. The key is to not continue to feed them, for if you do they'll just stay at the trough.
For these folks, continue to be sweet, but change the way you deliver help - for them there is a "special price" - full retail, and for them there is a specialty class with the answer to their every question. Soon they should move on to your competition and try to suck freebies there instead of from you - but they won't be able to say you were mean to 'em. There's a slim chance they'll morph into real customers, but that's unlikely, and getting rid of 'em is what's best for you and your shop.
This is a hard lesson - especially when what you really want is to share your love of diving, your knowledge and your enthusiasm - for everyone to love diving and to go diving with your shop. But if you lose the shop you lose the vehicle to accomplish the goal, and sometimes you simply have to fire losing customers.
E. striatus