ArnelGP's answer is valid as to the code. Your problem MIGHT be where to put this code, and that kind of depends on your setup. The description in post #30 of the thread is unclear and doesn't help.
If you implement a "test taken" table then a couple of things COULD happen.
If you put code under the SUBMIT function (I assume a command-button OnClick event), you can make the entry as I described so that in subsequent attempts to take a test, the same form's OnOpen routine can block entry by saying, "You have taken the test." And a FormOpen event can be canceled, which would stop the student in his tracks.
But, if you have a form to allow multiple different tests and the student has to select one, you would simply wait until the student selected the desired exam and do your test then. If the student has taken the test already, you would a DoCommand Close function to close the form.