Not all of these are going to be the most complex solutions in the world. Some superheroes are just flat out easier to take out with Iron Man's skill set than others, especially when - barring some advanced tech - they're just your average squishy human.
Since this is going by the version of Ant-Man that founded the Avengers in the comics, this means Tony is dealing with Hank Pym, who is a very different Ant-Man to go up against than the more well known Scott Lang.
While he is LEAGUES smarter than his inheritor, he's also far less adaptable, stuck in his ways, and generally kind of a massive stick in the mud jerk. As such, taking him out won't require nearly as much out of the box thinking as with Wasp.
Being a smart cookie, Hank's first instinct when fighting Iron Man seriously will be to - of course - shrink down and try to get into his armor to end the fight quickly. But since Tony is smarter, and knows Pym's tech inside and out, he'd booby trap his armor for Pym, stunning him long enough for Tony to shake him out of his armor. After that, it's as simple as just not watching where he steps.