There is no way to do this. Print orientation is something the USER specifies, via their print dialog box. You cannot and should not want to control this. The answer is to specify a print style sheet that formats the document in such a way that landscape orientation is not required.

This falls into the same category of developers wanting to "disable" the user's Back button. That's a no-no. That's part of the USER interface. The proper way to handle the situation is to assume they MIGHT press BACK, and write code to handle that eventuality.

You cannot FIX a problem by BREAKING user functionality.