You can close the file. The purpose of global fields is so they remain there.. What are you trying to do?
There is no one function built in that can do that. A script could clear all globals--I keep such a script in place on some of my hosted files as I can use a server script to run it and make a permanent change to the fields--a simple way to clean up the values I might have left in a field after working with the file off line.
I've also seen it set up that each global field was set up with an auto-enter calculation where the calculations alll referenced a value in a single global field. Changing the value in that one field then cleared all the other fields.
Do keep in mind that the behavior of global fields differ depending on whether or not you are hosting the file over a network as compared to what you get with a single user setup.