Disabling the Philosopher's Stone Ability

[DragonRX]

Hello,

I was wondering if anybody would be able to help me disable the philosopher's stone ability to change blocks on a tekkit server. I have tried using this in worldguard:

[5206]

ignore-perms=usage.phstone

ignore-groups=

on-use=notify,deny,log,tell

However it only tells the user that they cannot use the stone but it changes the block anyway.

Does anyone have any other ideas on how to disable this?

Thank in advance!