Something like this -

var lastScoreTime : float;
var score : float;
var pointLossPerSecond : float = 0.67;
var penaltyTime : float = 0.5;

function Score( amnt : int ) {
 score += amnt;
 lastScoreTime = Time.time;
}

function Update() {
  if ( Time.time - lastScoreTime > penaltyTime ) score -= Time.deltaTime * pointLossPerSecond;
  score = Mathf.Max(0, score);
}

Humm, try something like this:

static var Score : int = 20;
static var KilledEnemy : boolean = false;

    function Update()
    {
       if(KilledEnemy == false){
          Invoke("Timer", 5);
       }
    }

    function Timer()
    {
       Score =-2;
       yield WaitForSeconds(3);
       Score =-2;
       return;
    }

Hasn't been tested, in a hurry - sorry.