You should never use mothballs in the outdoor environment for any reason. They are harmful to everything...and will not repel the birds (who might even try to feed on them).

I's suggest that you do some looking around (on the internet, first, for some ideas) about squirrel proofing your feeding stations. There is a wide variety of (so-called) squirrel proof feeders, baffles, etc. Some can be very successful.

The active ingrediant in moth balls is naphthalene, a class 1 carcinogen so moth balls should never be used in any many not specifically outlined on the packaging, always in a sealed container. Besides it is a violation of Federal Law to use them other than prescibed.
Squirrels can be a real pain for us bird feeders, but there are ways to keep them from the feeders. First never put a feeder closer to anything a squirrel can climb and jump from than 10 or 12 feet. second be sure the feeder is high enough off the ground that they cannot jump to it (I have seen the wee buggers jump 4 feet to reach a feeder). Third if the feeder is held up by wire thread some 2 liter plastic bottles on that wire, drill holes just a bit larger than the wire so when a squirrel does get ont that bottle will simply spin around. Fourth if the feeder is pole mounted get a 2 foot section of 8 inch sheet metal duct, make a round wooden disk to put in the top of the duct with a hole just large enough in the wood to slip over the pole, after placing something on the pole to keep that duct up of the ground, and put the duct work on your pole. The squirrels will still climb up that pole but the duct work will keep them from getting to the feeder.
For one feeder I have what is called a "Twirl A Squirrel" a device that twirls the feeder around when a squirrel gets on it that worked quite well for a time until the squirrels found they could still hang on and eat all the while getting a ride.
There is also a, very expensive, feeder out there that has an electric fence like charger that jolts the squirrels (they make contact with two pieces of metal that the birds will not) so they get knocked off. Theoretically the squirrels "learn" to stay away from those types of feeders, but in reality not that I have seen. If food is there they will try everything to get to it.

On the subject of springloaded feeders -- the springs on mine got stretched out somehow, so now it won't open up at all, squirrel or no. Anyone have any idea if there's any way to get it working again?

If your feeder is mounted from deck railing there's not really enough height to protect it from squirrels jumping over any baffle you put under feeder.

Baffle 4-5' up from ground on 6-7' pole with pole at least 6' from nearest jumping off point 360 degrees around pole was our solution.

Nothing worked on deck...they could jump over & around any barrier that close to food source. Tried hanging feeder out on clotheline type thing strung from deck to house with 5' clearance on either side of feeder.

They are ingenious little scouts who quickly learn the "paw over paw" method of traversing the clothesline to get to the feeder. Hysterical to watch but waste of seed.