Hi Claudio,
Combination of financial transaction and category has very important meaning in FM process. Some commitment items are supposed to be defined in certain way if you want to secure correct update of FM flows. So, you have to see your case in particular why you would need no update in FM tables. What is functional requirement behind this request?
Setting CI transaction to 50 will stop updating FMIFIIT/FMIOI tables, but it this aciton has it consequences. So, before giving you a right advice, you first need to elaborate on your functional requirement.
Regards,
Eli