I know how maddening this is—squirrels chew to keep their teeth from overgrowing, and a wooden bench is basically a convenient gnawing block. The good news is you can stop them without hurting them.
Start with the spots they actually chew. The most reliable fix is to cover those areas with 1/4-inch galvanized hardware cloth or metal mesh. Cut it to fit around legs, armrests, or seat edges, then secure it with stainless steel staples or UV-resistant zip ties. Pull it snug and leave no gaps. Squirrels are persistent, but they will move on if they cannot get their teeth into the wood. You can paint the mesh black or brown to make it less noticeable.
Then make the wood taste terrible. I have had the best luck with a capsaicin spray: simmer cayenne pepper or hot pepper flakes in water for 15 minutes, strain it, add a few drops of liquid dish soap, and spray the bench once it cools. Reapply every week and after rain. If you do not want a DIY option, use a commercial bitter apple or pet-safe chew deterrent. Test any spray on a hidden section first to be sure it will not stain.
Next, add scent deterrents near the bench. Squirrels dislike peppermint oil, garlic, and predator scents. Soak cotton balls in peppermint oil and tuck them underneath or around the bench, but not directly on finished wood. Refresh them every few days because the smell fades quickly.
Also give them a better option. Place a couple of untreated pine blocks or squirrel chew sticks on the far side of the yard. If they have an easier target, they often ignore your bench. You can rub a little peanut butter on the blocks to attract them at first.
For stubborn squirrels, use a motion-activated sprinkler or ultrasonic repellent near the bench. The sudden burst of water is very effective and completely humane.
Finally, repair and seal the damage. Sand any rough chewed areas, fill deep gouges with exterior wood filler, and seal the bench with exterior polyurethane, paint, or a weatherproof wood sealer. This will not always stop a determined squirrel, but it removes the ragged edges and scent left behind from previous chewing. If the bench is already heavily damaged or you are replacing it anyway, choose composite, metal, or dense hardwoods like teak, ipe, or eucalyptus. Squirrels usually prefer softer cedar, pine, and fir.
I would use at least two methods together—usually metal mesh plus a taste deterrent solves it within a few weeks. Stay consistent and avoid poisons or sticky traps, which can harm squirrels, pets, and other wildlife.