Sticky traps at base of feeder, or near top, if feeder is near other vegetation that you don't want to trim.
If you have used a pole to hang a feeder, tie some bay leaves or mint leaves around the pole and also around the wire on which feeder hangs. These leaves act as a great repellants to ants You could also hang the feeder with a thin fishing line. Ants find it difficult to grip these lines.
I smear Vicks Vapo Rub on the bottom of the pole from the ground up to about a foot. I never have any problems. And it's far enough away from the feeder that it doesn't harm or bother the humming birds.