Detect collision with specified object

You can use something like

function OnCollisionEnter(Bam : Collision)
    {
    if(Bam.collider.gameObject.name == "whatIWannaHit")
        {
        //What you wanna do on collision with GameObject "whatIWannaHit"
        }
    }

However, if you want achieve something like 'only hits enemies', it might be more convenient to use tags. You can for example create a tag named "enemy" and tag all enemy GameObjects with that. Then you can check for the tag like that:

function OnCollisionEnter(Bam : Collision)
    {
    if(Bam.tag == "enemy")
        {
        //What you wanna do on collision with GameObject tagged "enemy"
        }
    }