What can you put on hummingbird feeders to stop honey bees?

The problem is the design of the bird feeder. Most H-bird feeders use an inverted bottle. As it is emptied, an air space develops over the nectar and this air will expand and contract as it warms during the day and cools at night. This acts like a pump to force more nectar out the bottom of the feeder than the birds consume so the excess pools around the feeder "Flowers" and can be reached by bees.

The answer is to discard your bottle feeder and get a dish feeder. I found that these are somewhat difficult to find in stores (8 varieties of bottle feeders and 0 dish feeders) so go online and look. You'll find many models so read reviews before making your choice.