This is in fact a vanilla command.
The exact command character for character, caps sensitive, that you want is:
gamerule mobGriefing false
The server gamerule system is a bit interesting in that it has the vanilla gamerules, but if you feed it another one, lets say:
gamerule thisisonlyatest true
Well, you now have a gamerule entry for that. And its case sensitive. So if you do feed it:
gamerule mobgriefing false
You now have two gamerules:
Which is the real one that actually disables mob grief, and
Which doesn't do anything at all.