@Karin:
If an enemy guard is immune to your attack, then you are free to attack any other creature you wish. This would fall under the same thought as the mandatory action rules in the rules supplement. If you are unable to perform an action, then you can not be required to perform that action. If you can not select a specific target for an action, then you can not be required to select that specific target.
@Borg:
Now, regarding the idea of changing immunity into an optional ability to avoid attacks, counter spells, or counter enchantments, I kind of like it. It would also give us something other than [mwcard=FWE08]Mind Shield[/mwcard] to make use of the Counter Enchantment step. I didn't think of the simple solution of making immunity into an optional ability. I'm not sure how popular this idea would be with others, specifically the Arcane Wonders higher ups, but it sounds intriguing to me.
Also, if we did implement something like this, there's no reason for Hydro Immune objects to be damaged by spells with Extinguish. Currently Extinguish breaks the normal immunity rules. There's no reason it couldn't break the new immunity rules as well.