The feeder is probably leaking, or is allowing the wasps to feed on the nectar. You will probably not going to be able to stop them as long as they can access the food. I would consider replacing it with a newer feeder. Newer models such as a Hummzinger, perky Pet and so are engineered to be bee and wasp proof. They may go to it at first, but when they can't get nectar they quickly give up.

Here is a link that might be useful: Feeders

I have a bee guarding my hummer. I purchased at the dollar tree so yes it's leaking:-(

Now that I know I enjoy the birds and my kids do too, I will buy some better ones for next year.

Yes wasps can easily be drawn to this kind of leaking feeder. Try to find kind of saucer shaped feeder [hummzinger style] and that will or should eliminate your wasp problem.