Mega Stones onTakeItem: false,

Mega Stones are confirmed to be unremovable, even by Knock Off. http://www.smogon.com/forums/threads/pok%C3%A9mon-battle-mechanics-research.3489239/page-3#post-4876344
Merge pull request #390 from ALAKTORN/patch-2

Mega Stones can't be removed