I had an issue with squirrels getting into the eaves and attic of my house.  they had chewed holes through the cedar facia boards.
I tried a number of various repellants to discourage them, but nothing really worked.  They were actually tearing up the fiberglass insulation to make nests, so i don't know what it would take to really irritate a squirrel.
I finally bought a live trap.  I used a peanut butter and bird seed mixture as bait.  Each morning I would load the trap and each evening I would release a trapped squirrel into the woods west of my house.  The advice I found on the web suggested that the squirrels have to be released at least 10 miles away so that they don't find their way back.  After a couple 
You are never going to build a catapult that will fling them ten miles away!  

After a few weeks of trapping the squirrels, we had the soffit and facia redone in aluminum.  they have not come back in since.