Hummingbird feeders vs. bees

Does anyone know of a fool-proof way to keep bees away from hummingbird feeders? Mom has a bee guard on her feeders but they don't seem to deter the fellas. She doesn't want to hurt the bees but since they're running her hummers off, she's a bit upset. (I found mom out on her balcony today trying to swat the bees away with a fly swatter! Oh my!)

The bee guards were the first thing I was going to mention but...... wish I knew what to tell you, the bees dont seem to bother with my feeders at all. Someone should show up soon to help give you some ideas on what you can try.