is it already done fermenting?
are you planning to rack it to another container soon?
I have heard some people have used marbles to take up some volume, but i would be worried about dropping them in a glass carboy. plus you would need to rack to the carboy with the marbles(sanitized of course), not drop into the carboy with the mead in it.
maybe someone else has an idea, I would like to know too.
cheers