In virtualbox fullscreen mode, can I disable or move the popup menu bar?

You can disable the menu bar. For the setting to take effect you may have to exit and return to fullscreen/seemless mode. No need to shut down the virtual machine first.

Go to the settings of the machine. In "General" on the "Advanced" tab you can select: "Show in Fullscreen/Seamless" and there's an option to "Show At Top Of Screen".

In VirtualBox 5 the setting is under “User Interface” instead of “General”.

You can change the settings without shutting down the machine. Just go to machine > settings.

For the the menu bar settings to take effect, you will need to leave full screen mode and then re-enter it

The option to display the mini menu at the top is now (I am running version 5) in the User Interface section rather than general. I would post an image of the dialogue box - but I don't have 10 reputation points!