Make trigger so if hero that isn't special picks an item he drops it and displays a message "Can't use that item". If u can't handle it tell and I'll post u exact triggers.
EVENT:
a unit aquires an item
CONDITION:
(item being manipulated) equal to (your item)
ACTIONS:
if-then-else
-if (triggering unit) not equal to (your special hero)
-then drop (your item)
message (only xxxx can use this item)
-else
hm.. sorry that i cant give more exact for now. if something is unclear post again.