WHAT YOU HAVE TO DO Multi-family dwellings are a tough call.  You'll have to attend to YOUR unit, all by yourself, if you can't get the co-operation of ALL of your neighbors.  If you can get that complete co-operation, it will be much easier for you to eliminate the problem.  At which point, to do it the RIGHT way, you'll have to begin (IMMEDIATELY) an active campaign of trapping the squirrels that are entering the building(s), best by doing it on the inside, (no innocent bystanders) and out of the sight of ‘pesky' neighbors. WHAT TO DO You trap all that are entering, Then, after you are SURE you have them all, you seal off entry points with hardware cloth.  And not the cheap stuff you get from Home Depot, you get the HEAVY-DUTY hardware cloth from the hardware store.   If you have the co-operation of all your neighbors, you do this on the outside, patching all points with the hardware cloth, backing up each patch with newspaper.  You do it this way so you can tell if there are any squirrels that are left inside and seeking an exit route.  (They will remove the newspaper gag to get to the outside - you will be able to see that from the ground, if you do it correctly.)   NON-CO-OPERATION If you have anybody that WON'T co-operate, you'll have to do this from inside your attic.  You do this by, more or less, trial and error.  Seal off all the points between you and your neighbor's attic.  Use the same hardware cloth I specified above, use it at the firewall dividers and block off those small areas where they are often able to slip through.  An arduous task, for sure, but it will be your only way, along with a continuing trapping campaign, to keep these interlopers at bay.   KEEP THOSE TRAPS Make sure you always have operable, appropriate traps at the ready, to take care of any problem as it arises.  I would KEEP them set and baited, ready for any squirrel that enters your attic in the future.  Let that trap be his welcome.  Use a ‘bait' that won't degrade, such as fuzzy material or cotton balls, which they'll always scrounge for nesting purposes anytime.