Additional ways to deter Squirrels from feeding from your wild bird feeders are:
Ensure that the Squirrel cannot access the feeder by jumping on to it from nearby trees or climbing up a feeding station etc.
That as an extra deterrent to the Squirrels getting onto a feeder a tree hook is used as this will increase the distance between the feeder and the hanging point. It may be worth covering the tree hook in Vaseline as a preventive measure.
Provide a Squirrel Feeder for the Squirrels as this may encourage them to feed from their own specific feeder and leave the wild bird feeder alone.
Also, the RSPB suggest mixing in with bird seed chilli powder as Squirrels do not like the taste of this product while bird do not mind it.