No, that’s not what flatten is for. Flatten is only for controlling the mesh of the ground.
Polygons and Exclusion Rectangles are for excluding items. I would try both, as the pylons are library items, so it’s possible they can be excluded with a polygon. If they can, they’ll disappear live. Otherwise you’ll need an exclusion rectangle.
Exclusion Rectangles can be rather tricky to use right. I was deleting some parking lot cars, and one set (there were 3 parking lots next to each other) refused to go away until I found the right size for it. It wasn’t obvious how far I needed the rectangles I was using to extend around the parked cars, but, eventually it worked. I’d figure out if it’s possible by using an Exclusion rectangle and make it as big as ALL the towers that have been placed in that line, and have it exclude everything. Then, if it works, turn off options till you figure out which one worked. Then split it up into a bunch of smaller exclusion rectangles along the line of the towers and see if you can get that to work so you don’t exclude too much.