So. For a lot with the spleef flag set, I suggest two commands be added: /spleefsave and /spleefload.
/spleefsave will save the block contents of the current spleef-flag-set lot.
/spleefload will reset the spleef-flag-lot to the last state that /spleefsave was used.
This will allow the easy resetting of any spleef arena, no matter the shape or size. It will also allow a Fort World type of thing, which had pretty much unanimous and great support as a suggestion.
Because the spleef flag is set, no blocks will drop, meaning that there is no way to duplicate blocks or otherwise abuse the feature.