Fling to dismiss windows
VR with hand presence should be a very natural process. Manipulating the environment directly rather than through button presses should be the end goal of any UX design in VR.
With that in mind, I'd suggest allowing windows to be dismissed by grabbing and flinging them away, or at least dropping them in or flinging them toward a trash icon. Pushing a button is fiddly, and while there are other suggestions (e.g. pressing another button while holding the window) I think the more hands-on approach would feel more natural.
Obviously this needs to go alongside other methods for non-Touch configurations.