Great question from someone who obviously understands metal.
For thinner metals you use this accessory (see pic) called a "stripper." In the pic below the white masking tape is holding the screws that secure it in place. Basically it attaches to the guard for thinner sheet and narrows the gap on the hold downs to limit the spring back effect. Mine (shown) is the basic version; but you can get deluxe versions that feature adjustable spring tension against the sheet to further protect from spring back. There is actually a formula to calculate the spring back counter force on the hold down needed to eliminate distortion. Practically, I've found if you need such a device you're probably using such thin gauge sheet that you don't need a 25 ton ironworker to begin with—let alone a 100 ton model. You can easily punch 26 gauge by hand (or almost by looking at it if you're as ugly as me). Not sure if you can see it in one of the pics, but the four bolts holding the guard/hold down in place are big: 3/4" in fact. I was surprized to find how much force is involved with the pullout of the punch for thicker stock. Thin stuff is nominal though.
Using my basic stripper I've punched 22 gauge with almost no distortion (see pic of curved part with round and oval punched holes after powder coating). I've heard of people punching 26 gauge with the deluxe strippers, but I suspect if I crunched the numbers that would be a waste of power to work the hydraulics on an ironworker. Industrial machines such as you see on TV programs like "How It's Made" do multiple operations at once and are different. Unlike a drill press, you should never stack metal on an ironworker—unless you like to break punches. That's expensive, and from experience I now appreciate why they put guards on these machines in the first place. Breaking a punch is a Depends Undergarment moment. Bloody frightening.
Personally, I've found that you need to be creative and fabricate your own jigs and hold downs to accommodate spring backs on thin sheet. I use Clecos and different clamps to do this. Sure, there are custom devices for this...but if you can't fab your own, I'm not sure you should be using these machines to begin with.
A few other tricks to avoid distortion include wiping the punch occasionally with oil or lube, and—for thinner stock—using cold rolled steel instead of hot rolled. Cold rolled "snaps" distinctively when punched and sticks less to the punch on pull-out. Hot rolled "sticks" (or grabs) to the punch more, as well as inside the die, and then requires you to lift the part and pull it out due to the downward ridge left in the die. I always request cold rolled sheet for punching. But sometimes metal suppliers mess up. I know immediately because there is a distinct sound difference on an ironworker when punching cold rolled vs. hot rolled. It also increases cycle time on production runs I do, and the ridge means instead of sliding the part out quickly, you need to use pliers to pull the ridge out past the die. That adds time to the production run, and is a PITA. I actually did a blind test where I mixed cold rolled vs. hot rolled in a bunch of 16 gauge steel. I surprized myself and could see the difference both in the distortion and the sound.
Took me five years to figure this all out.