foreach (Button button in this.Controls.OfType<Button>())
button.Enabled = false;
Edit:
You may actually need to do more than this. The Controls collection only fetches the controls that are the immediate children of a particular parent, and it doesn't recursively search through the entire page to find all the buttons. You could use something like the recursive function on this page to recursively find all the buttons and disable every last one of them.
If you add the code from the above linked page, your code would then be:
foreach (Button button in FindControls<Button>(this))
button.Enabled = false;
These kind of recursive methods will come in very handy in ASP.NET once you use them a couple of times.