MM AUTH: Account assignment in ME21N

Question: Hi!
Any one knows an object to restrict Account assignment category when creating a purchase order with transaction ME21N?

Some users shouldn't be able to assign type A (assets), but all user should be able to assign K (cost center).

I though it could be controlled with F_BKPF_KOA, but it seems it doesn't work.
Any other object that can help? Or should I start looking for a user exit instead?

Answer:
Duplicated here...
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