As a last resort set out a rat trap baited with peanut butter. BTW, squirrels are excellent eating.
Take care in using rat traps as they will catch the bigger birds that you are trying to promote. Not only that, but for the most part, they don't have enough power to kill the squirrel. I use these traps for Grackles and Starlings, when they over-run the feeders. The bait you use, is important....
Squirrels are very smart and probably the smartest game out there but they have a very short memory. If you lay off for a time, they will forget about you and come in, again. .....
Be Safe !!!