Thank you for your question.
The answer is that you have two options:
1. Write a letter to the Belgian embassy where she deposed the documents, entitled "Opposition to Marriage", in which you should say as follows:
-> your and her name and identification details (birth date, address);
-> the fact that she submitted documents in order to make a marriage application;
-> the fact that you consider one of the documents to have been forged;
-> the fact that the marriage application would have been a "mariage blanc" which means a marriage for the sole purpose of obtaining a residence permit in Belgium;
-> the fact that in any case you declare your opposition to being married to this person;
-> your name, date, signature and, if you wish to be extra-safe, the name of a witness who can attest to the truth of your letter.
2. Besides this, simply absenting yourself from any marriage preparations / ceremony will suffice for the marriage not to be completed. One cannot get married without being present.
In any case, a letter as above will guarantee that the civil status officer will deny her application.
I hope my answer was useful and look forward to your rating, which is essential to my activity.
Dr I L Vlad