How can I turn off MobGriefing but only on one item?

So I was playing with my friends on a modded server and they wanted to steal some items from a dragon… This, of course, didn’t get as planned because seemingly sleeping monster woke up and set everything on fire. One friend’s stuff didn’t burn because one mod – erebus – adds graves to the game but second player wasn’t that lucky – dragon broke his tomb and all his items are gone now.

And here is a question: how can I make graves indestructible for mobs but destructable for players?

(basically turn off MobGriefing on this item only)

Informations you may need:

• Mod for dragons: Ice and Fire
• Mod for graves: erebus
• Grave id: erebus:block_of_bones
• Server software: Magma

Thanks in advance,
Jaro975