The problem is that squirrels not only chase away the birds that are trying to eat, but that they tend to destroy the feeders themselves. If the feeder is too slow in dispensing the seeds for the squirrel, they will chew the opening to make it wider. Sometimes they will chew new holes into the sides of some feeders.
If you really want to make your bird feeder impenetrable to squirrels, you have to cut off every access path they have. That means two things: making the mounting pole impossible for them to climb, and placing the feeder out of jumping range.
When positioning the feeder, you may need to keep the feeder as many as 8 to 10 feet away from objects they could use as launch pads. This includes bushes and tress, of course, but other things like picnic tables, birdbaths, etc. Also, be aware that if there are branches above the feeder, the squirrel can drop down from seemingly insane heights to land on the feeder.