How to stop a running function when another function is fired

Question

I want to pause/stop a running function below when the other function is firing.

function log(msg) {document.getElementById('log').innerHTML = msg + '<br/>';}

var a = function() {
    var j = 0;
    setInterval(function() {
        log('A: ' + j++);
    },1000);
}

var b = function() {
    var j = 1001;
    setInterval(function() {
        log('B: ' + j++);
    },1000);
}

var b1 = document.getElementById('button1')
  , b2 = document.getElementById('button2');

b1.addEventListener('click', a, false);
b2.addEventListener('click', b, false);

Also, I want to prevent these functions from double fired (or fired multiple times) if I hit a button repeatedly.

Thank you,