You cant really prohibit units from picking up certain items, but what you can do is force the unit to drop the item immediately.
You write a trigger which checks for (event) "unit aquired item" or something along those lines. You use a condition which compares the type of the item to the item type you want to limit. You also check the units type.
If the item was picked up by an inappropriate unit type you force the unit to drop the item. There is an action in the "Hero" category which does that.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory