Unless it got fixed in recent updates, they don't continue spawning while you are on top of the big tanks and such. One of them has a ladder you can climb, or you can climb the boxes. You still have to kill the full number of zombies, but it lets you control when they spawn. Furthermore, when you are jumping between tanks, it seems you are briefly counted as being on the floor below, so you can spawn one (1) zombie at a time by jumping between tanks, and repeat until done. Just be careful about the maniacs (runners), as they will jump onto the tanks with you and you'll take damage if they bump into you during the jump. They're easy to take out while they're recovering from the landing though.
This is totally an exploit so it might feel a bit "cheap", but I don't mind personally as I find the spawning works to be kinda BS to begin with: they spawn continuously in places near you, rather than in specific locations, so if you were to try to run away, no matter where you went, you'd find new ones spawning right around a corner next to you. So it's not usually possible to do a "tactical retreat". It's not as bad in this case, but in the Hangar challenge mode, there are places where they will literally emerge from behind a tree like a cartoon.