disable close Acrobat X in application (allow only File, Exit)

I'm using Access VBA to work with Acrobat documents. The only problem I'm having is that if

1. VBA creates an object that refers to Acrobat, then

2. the user closes Acrobat,

3. VBA tries to access or open Acrobat

Acrobat crashes. I haven't been able to come up with a way for VBA to test whether or not Acrobat is already open, etc., like I can do with other objects.

I'm using:

Dim appAcrobat As Acrobat.CAcroApp

Set appAcrobat = CreateObject("AcroExch.App")

It would be good if Acrobat were to stay open all the time, which I'm trying to train users to do. I.e., have my application close documents, and have users close documents (instead of closing the application).

Is there a way to disable the "X" in the top right corner of the Acrobat application, so users won't close the application? Then I could tell them to use File, Exit to close the application.

Thanks.