Whenever Juggernaut attacks he cannot ready this turn. Does that include when an effect like Reset is played to remove a defender?

Yep. If he ever starts an attack, he can't ready for the rest of the turn, period.

Short of Bodyslide or the like, once his effect triggers, he's stuck unless you can ready him in response or find a way to prevent the effect from resolving. Only Human on him would be techy, BTW.

another fine use for reset... What doesn't it do?

Take sexy photos?