try setting the triggerzone's main to this
;Artificial Intelligence Script
;Header
desc = Plr In Zone, activate entity specified in IF USED
;Triggers
:state=0,plrwithinzone=1:activateifused=1,sound=$0,state=1
:state=1,plrwithinzone=0:state=0
Then give your rock this
;Artificial Intelligence Script
;Header
desc = rock
;Triggers
:activated=1:destroy
;End of Script
Or if you have 1.15 you can try this
;Artificial Intelligence Script
;Header
desc = rock
;Triggers
:activated=1:addhealth=-9999
;End of Script
Make sure to set the ifused in the triggerzone to the rock