I don't really understand why you can't attack beguiled enemies. It's really not fun to have a beguiled enemy ignore every attack you make, especially for solo battles. I'm wondering if it's a bug that happened because the enemy turns into a teammate, or if it was intentional for some reason.
Re: Why can't you attack enemies that were beguiled?
I believe this was intentional for the reason you stated. Since an enemy turns into a teammate, you cannot attack it in the same way you cannot attack your own teammates. Beguile works the same way when enemies use it on your wizard. They cannot attack you for a round.
Re: Why can't you attack enemies that were beguiled?
Beguile means that you've tricked your enemy into helping you for a bit (or your enemy has tricked you!). While he's beguiled, he's on your team. While you're beguiled, you're on his team.
Why would you attack your own team - see?
So, in order to attack and kill, you should wait until the beguile passes. Then you're enemies again and you can freely kill each other.