As the saying with mice, or cockroaches, or any pest goes, if you see one you got a bunch more you can't see. Aiptasia is one of those things that should just go into the same category as all the other mundane chores you do on your tank. Big ones are relatively easy to spot, but unless you have a sharp eye you can have a bunch of little tiny buggers that don't register as aiptasia to you. For me I tried to make sure when putting rock up, no rock was more than 3 rocks deep, meaning if I saw something bad on a rock on the bottom, moving 2 rocks got me to the rock I needed to remove. And every time I see an aiptasia, I pull the rock out, grab some cutters, cut off as much rock around the aiptasia as I feel is necessary, or if I scrape it off I give it a nice super glue coating. And back in the tank.
If my tank got super infested like one of those pictures, I'd probably break everything down and be done with the hobby though.