Hello Karina,
The events that you are looking are classified under the Infosystem (P/S) in FIBF but these cannot help in stopping the action that is being performed, Please look for a BTE event in Infosystem (Processes) category to make necessary changes to the document posting.
I dont remember the BTE event exactly, but there is an event that will have exceptions along with it, and you can make all the validations and if the validations fail raise the exception to stop the document from posting.
Regards
Selva