Squirrels can be a pain, they destroyed two of my own feeders in days. I bought an all metal one made of stainless steel which has lasted years now.
RSPB website has some anti squirrel feeders which should give you some idea of what to look for . Most involve cages around the feeder, allowing small birds in but keeping squirrels out.
If they are greys (and at the risk of upsetting some members of the site) I would suggest that you buy or borrow an air-rifle. Greys are nasty vicious things that have virtally elliminated our native Reds and are actually classed as vermin. I know some people will disagree as this subject has been discussed several times on site, but when all said and done no matter how cute they look they re nothing but tree rats and most people have no qualms about getting rid of rats
If you have one of those proper feeder tables where the vermin can not jump on the table from above/side, nail a 12" high piece of tin halfway up around the pole so they can't climb up it.
I've stopped the squirrels clinbing up the pole my feeders hang on - smear oil or butter on it and then watch them try and climb up - a laugh a minute as they slide down again.
My hubby got an empty plastic bottle cut the bottom off and fitted over the feeder after measuring the bottle and cutting it so that it only enabled nuts to drop to just below the cut off bit. We found the squirrels couldn't get a grip on the botttle