How do I pause the installation of iOS apps?
To pause the download or installation of an app you need to tap once on the icon of the app in question. To resume the installation you need to tap once again on the same icon.
To completely cancel the installation of an app you need to tap and hold any application icon on your home screen/springboard until all the app icons start to wiggle. Then you can delete the apps you want. (The ability to delete apps that aren't completely installed yet is only available since iOS 5)