Hello Claudio,
standard should update the field ZLSPR "Payment Block" only for document item (BSEG) for Vendor Line, not the rest of the document... is your situation that even after setting the Payment Block in MM Invoice (RBKP), the FI item for Vendor (BSEG) is still with empty Payment Block? That is an issue it itself, since such document is not considered as blocked by payment run; weird.
Anyway, if you need to change the FI document content, FI Substitutions (see transaction OBBH) would be the correct tool for this.
Kind Regards
TomT