Solution #4 - change the handle

A variant to the previous solution would be not removing the click function, and rather assign a new one instead. We are using functions as first class objects here, when we assign the alreadyBilled() function to the click event:

function alreadyBilled() {
    window.alert("Your billing process is running; don't click, please.");
}

function billTheUser(some, sales, data) {
    document.getElementById("billButton").onclick = alreadyBilled;
    window.alert("Billing the user...");
    // actually bill the user
}

There's a good point to this solution: if the user clicks a second time, they'll get a warning not to do that, but they won't be billed again. (From the point of view of the user experience, it's better.) However, this solution still has the very same objections as the previous one (code coupled to the button, need to reset the handler, harder testing), so we won't consider it to be quite good anyway.