disabling a button is an html feature and you can't really control that from app lab (v. css which you often can).
A clever (hacky) way to get the behavior you want without variables is to show/hide a label (with no text) that sits over the button. When the label is showing the button cannot be clicked. When it's hidden you can.
Here's an example where I made the labels red so you could see them.
Make the labels white and/or semi-transparent, boom, disabled button.
An option to consider. Cheers,