Keeping squirrels away from bird seed has always been a challenge.  A popular choice these days is to select seeds blended with hot pepper or suet with hot pepper.  The birds are not bothered by the hot pepper but it does effect the squirrels, keeping them for emptying your feeder in one visit.  The hot pepper seeds and suet are more expensive but can be cheaper in the long run if squirrels are eating much of the seed you are putting out.