In a technical sense we can't really forgive someone who does not repent. But that doesn't let us off the hook. I think that God requires us to love our enemies, which on our end of the transaction differs little from what forgiveness would require of us. So technical forgiveness is not the issue, really. It is love from the heart, that is what God requires of us toward even our enemies.
Forgiveness is what flows from that kind of heart when there IS repentance.
Love must always flow. Not that any of this is easy. Often it is tremendously difficult. But discipleship is following and learning from Jesus. If we fail to learn this lesson of forgiveness and loving our enemies, we miss the essence of God himself.