Have you ever purchased a squirrel proof bird feeder that isn't really squirrel proof?
The feeder I purchased is actually squirrel proof, but only as far as the squirrels not being able to cause damage to the feeder. It did little or nothing to prevent the squirrels from eating the bird seed.
The squirrels would just hang upside down to grab the seed or push it down to the floor. Sure this would slow them down a little, but it did not prevent them from emptying the feeder without much effort at all.
This simple modification slowed them down quite a bit. Now my bird seed will last 3 - 4 days, instead of literally 3 - 4 hours. Sure they still hang around and keep trying as they are very persistent, but they don't get much seed anymore, if any at all.
Simply wrap a piece of the hardware cloth around the base of the feeder and fasten with nylon ties. This will prevent the squirrels from reaching the seeds which have fallen to the bottom of the feeder.
Before this simple modification, some of the more clever squirrels simply scooped the seeds on to the floor. They would then just climb down for a feast. Now they usually just give up after several minutes of frustration.
One more comment, I used to occasionally find this feeder on the floor. This was because the squirrels would cause it to spin every time they jumped on and off of it until it eventually just unscrewed.
A swivel hook solved that problem: