If frenzy requires you to charge an enemy model that is in range, but you don't want to charge (for whatever reason), and you do all declared charges before forced frenzy charges, technically it's possible to declare a charge with the frenzied model, fail that charge, and keep him from charging the other model.
This would be useful when you have a model that is frenzied and the enemy is purposefully trying to drag him into a trap by putting a giant rat or wardog in the open. This is especially important if the frenzied hero has the Sprint skill.
So, is failing a charge (technically on purpose) legal?
I put this in the same category as actions that are technically legal but against the spirit of the game, so if it became common place you'd have to houserule against it. Kind of like when someone with a sling declares they're going to shoot a wall or other terrain object twice (they just want to measure to see if the object is within 9" which of course gives them a reference for the actual distance to that object or nearby models/terrain). For the shooting thing, we ruled that a player could declare any opponent's shots within range/within half-range without measuring at his discretion. This keeps players from 'pre-measuring' distances.