Best way to restrict items

What is the best, most fool proof way to have a hero only being able to pick up a certain item at a time. Say - Paladin already has boots of speed, sees another pair, but I want to restrict him from even picking it up. I know of 'Issued Order' but you can just keep clicking and you'll eventually pick it up. I don't want to waste the players time by making him think it's a bug, I want a direct warning. Thanks in advanced.

Here's the way I would handle it:

EVENTS:
Unit - A unit owned by Player 1 (Red) Acquires an item

CONDITIONS:
((Triggering unit) has an item of type (Item-type of (Item being manipulated))) Equal to True

ACTIONS:
Hero - Drop (Item being manipulated) from (Triggering unit)
((add warning message here))

Of course, if some duplicate items are allowed you will need to use the following type of IF/THEN statements in your ACTIONS. One for each type of item you don't want duplicates of...

ACTIONS:
If ((Item-type of (Item being manipulated)) Equal to Boots of Speed) then do (Hero - Drop (Item being manipulated) from (Triggering unit)) else do (Do nothing)

Vraq your trigger wont work as you think, read this.

Ouch, that sucks. I hadn't realized that, thanks for pointing it out!