This is a browser feature and is intended to protect the user, If you could turn it off, all those sites spamming users with dialogs would have a way to stop it.
So in short, no.
If you are not satisfied with the above answer and you want to go ahead and disable, then also you have an option - this is CodeProject buddy ;-)
Steps to recreate:
Alert2 is displayed with that prevent this page from creating additional dialogs annoying checkbox
Now change the above code into
Now you wont see that annoying message and its checkbox in the second consecutive alert
Dont reduce the timeout value, if its with in 1sec then browser will automatically add that warning to the alert.
Hope this helps you a bit.