Hi Rodrigo
If you do a search back on the forum this is covered on a few occasions, the most usual approach is to implement 2 authorization objects for P_ABAP and depending on the status you choose, it means that expese reports cannot be created in certain status for operation NEW and this is probably the cleanest approach to tackle this.
I havent finished this wiki page fully yet but the general idea is outlined here: (it is for "approved travel request" but you get the general idea!)
http://wiki.scn.sap.com/wiki/x/xIOTF
You mentioned "posting travel expenses" in your title but I presume you mean instead creation of travel expenses but preventing an expense report from being posted is always going to be challenging as the settlement program and PRFI will always select expense reports with status "settled".
cheers
Sally