clearTimeout()

js/script.js

var stop = null;
function funString() {
var str = 'Hello World!';
document.getElementById('p').innerHTML = str;
}
function funMake() {
stop = setInterval('funString()', 2000);
}
function funStop() {
clearTimeout(stop);
}
window.onload = function() {
document.getElementById('b').onclick = funStop;
}

var stop = null;
function funString() {
var str = 'Hello World!';
document.getElementById('p').innerHTML = str;
}
function funMake() {
stop = setInterval('funString()', 2000);
}
function funStop() {
clearTimeout(stop);
}
window.onload = function() {
document.getElementById('b').onclick = funStop;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<input id="b" type="button" value="Stop" />
<br />
<p id="p"></p>
<script>
funMake();
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<input id="b" type="button" value="Stop" />
<br />
<p id="p"></p>
<script>
funMake();
</script>
</body>
</html>