How to canel a button press in another application

Hi. I currently have a program that sits in the system tray that displays one of 3 icons.
I use it in our office to show whether a user can close invoices / close certain invoices / or not close invoices.

I know how to use the api to get text,press buttons in an external app, but i am now trying to find a
way to stop a button in an external app being pressed ie Not let a user close an invoice.

i have just seen this like http://www.vbforums.com/showthread.p...nother-process
and it appears to monitor system messages.

Does any one know how to take this one step further to see that a button has been pressed and then sop the message
being processed. i can then pop up a msgbox in my app to say closing of call not allowed at this time.

hope someone can help