Squirrels are tough to stop even with a treadle feeder.  A PVC pipe feeder like the OP has, no chance of keeping them out.  With a treadle feeder you need a narrow and distant treadle, not  a wide step, and the door MUST be spring pre loaded to provide some resistance to just being pushed open.  Most treadle feeders are missing both features unfortunately.
But squirrels are rodents so fall back on Howard E.'s posts on rodent control's three steps.  Sanitation, exclusion, elimination, in that order.  Do the first, you won't need to do the second, do either the first or second and you don't need to do the third method of elimination.
Sanitation, bulk feed in metal barrels with lids, a proper treadle feeder with the narrow and distant treadle and the pre loaded door, no plastic either.  Clean up all approach areas to force the squirrels out into the open for travel so natural predators have a shot at getting at them.  Squirrels will have  a territory and will travel several hundred yards to a shared resource like a chicken coop.  They will protect their own territory from other squirrels, chicken coops and farm lots tend to be shared especially if you have forest surrounding you.
Exclusion, Ft. Knox coop.  Hardware cloth all sides and buried deep in the ground.  Doors kept closed so no free range.  This method is far more expensive than the sanitation method but it can work.
Elimination, traps, playing Elmer Fudd, poisons.  A never ending chore and expense with short term results.  Empty territories quickly attract a new critter as young are born and then eventually driven away from the nest. Young males will travel looking for empty space or a territory where they can fight and win.
If you have swarms of squirrels they might overwhelm even a good treadle feeder but the feeder should also trap the squirrels and have   a french cleat so you can just lift the feeder up to dispose of the squirrel.  Check state wildlife regs before taking one for a ride.  Generally marauding  wildlife is okay to destroy on site out of season but again check with the local game warden.