Is there a way to close all application in the taskbar at once?
No.
You have to manually remove each one.
One nifty trick I learned from a friend: Enter "wiggle" mode for the taskbar, then tap the icons from right to left. This way you can close at least four apps in a steady motion without having to wait for the icons move one column to the right. Sounds obvious, but try it out and you’ll see what I mean.
