Right now it can only be changed from within the module, in a private variable.
This should probably be configurable. That way wikis can choose to disable them earlier, or perhaps keep them longer.
Although I believe this was requested before, I came accross this when working on the QUnit test suite. It has a mode in which it will error if a global is implied or introduced. It goes haywire currently, I'd like a way to turn it off when using that mode (wether or not we should do that should be discussed under bug 28915).