Disable Custom button after clicked by Javascript

After clicking of a custom button on detail page lay-out, Chances is there to click the same button. 

If your button Behavior is "Execute JavaScript",
Add following code to disable the clicked button when it is performing certain action calls.

Sample Java Script Code:

this.className = "btnDisabled";
this.setAttribute("disabled","");

After getting the response from action methods you can enable it again.