Item System help

I want to know how you can set it so that when you have one type of item, you can't pick up another.

Ex: you pick up a sword, and behold you see a better sword on the ground, you go to pick it up, but you can't. Why? because you already have a sword, so you need to drop your sword to pick up the new sword.

if any one knows how to do this, or direct me to and already established Tutorial/system map, i would be most greatfull.

I think there is one system in RAO DAO ZAO's map WhenTheFreedomSlipsAway
Ask him i think he can help you

ok ill see if he can show me that

ok i found his tut on how to do this on Wc3Campaigns