Without such a button, you could disable or uninstall the touchpad driver. Or at least turn the sensitivity/response settings down to flat minimum. The laptop would be difficult to use without a mouse, though.
I agree that it's a good idea and would be a useful feature. Accidental touchpad contact is fairly frequent when using the keyboard (especially since contact isn't always necessary, mere proximity can be enough), I've often experienced the same problem. I really hate it when I'm typing and (even with deliberate effort to lift my palms) the touchpad randomly yanks the cursor all over the place or moves focus out of the window I'm working with.
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