Is this possible? Modal dialog that user cannot just kill?

Hi - very brief summary first so I don't bore you:-

is it possible to popup a modal dialog (that re-centers itself on the screen) and is difficult (if not quite impossible) for the user to close down?

Some more detail:-

I've been asked to look into writing an Applescript that will auto start when the users logs in and check they are up to date on a particular task. If they have not I need to popup a dialog box (on top of any other window).

Initially this will just be a reminder - but after a certain number of 'nags' and they still haven't done the work I need to make the dialog modal, keep itself centred (i.e. move it back if the try and move it out the way) and don't allow them any easy way to close it until they choose to launch the app that I'm waiting them to complete their work in!

I'm completely knew to apple programming so please treat me like the noob I am! I am reasonably familiar with Unix and shell script (so I understand a user could 'kill' the process if they knew what they were doing!).

Thanks very much in advance!

Jonathan