The short answer is, you cannot. I find it weird why Google did not include this option. The Nexus 7 home screen by default is set to 5 and I really have no need for so many screens.
Considering there are folders to help manage your apps, this option should never have been excluded unless Google did this on purpose to force users to spread out all app icons on the home screen then that would make folders have no use at all.
So anyway, if you really want to remove the other home screens, the only solution is to download some other launcher like Apex or Nova.
There are free versions of them and you can already manage the home screens with it. There is no need to purchase the pro or paid version in order to be able to manage home screens.