I'm not certain that the AI is REMOVED when crossed by a human player. What I THINK happens is this...
when a human player has units ON the ai block, it takes precedence over tha block. SO, if an AI unit is in range, and set to defend an area, the computer will move its units and attack the player stack on the hex. Then depending on the AI and whether or not there are other units to attack, the indies will either (in THIS case of defend area)
1. ... continue to move on to attack the next player units within range, or...
2. ... move back to their original square and await the next set of units to attack.
Now, if the AI is set to say, berserk, again the indies will attack a player stack in range that is sitting on the ai block. once the combat is finished, if theindies live, they will continue along their movement path, thus effectively bypassing the ai block. This would be the case in other AI behaviors that allow the indies to attack the players without returning to their guard square.
It should be noted that if indie untis set to defend area cross the ai block, and the attack other units in range, they may efffectively be cut off from their starting defending square, unless there are more player units on the AI block to attack so it can cross again.
This will also effect the indies ability to defend it's initial area, since the condition is set to defend from that centralized starting point.
like I said, this is what I think happens based on maps I've created and problems I encountered. but I DON'T think the AI block just disappears, I think it is just effectively taken over by player units stepping on it. of course, I could be wrong :-P