well first of all, most people dont like it when the hero is given to them at the first click... they want to see their hero stats... I don't want to play a map the first time and select a hero without knowing the stats or attributes, etc. all you have to do is do the same thing as what qwerty or angelusz said, but at the end of the trigger, create the action, "trigger - turn off this trigger". This also means you have to create a hero selection trigger for each player. Also, to prevent other players from selecting a hero that someone else already selected, put this under condition, "selected unit is owned by neutral (or which ever player owns the heros in the first place) equals true". This prevents player red from selecting a hero, and then player blue to reselect red's hero, then red has no hero because his trigger is tunred off to prevent reselection. hope that helps.
-Blue