At first glance this sign looks to be an exhortation to all who happen by with writing implements. Upon closer reading, it appears as if the building owner is addressing a specific person or persons. Note that the sign writer said “I promised”; ‘promised’ is in the past tense. The building owner already promised not to write on the walls of the others. If that is so, the sign is a message for those whose walls on which the building owner promised not to write.
Rather than speculate on the possible twisted relationship between or among the wall writers (or the refraining wall writers) I will just assume that the building owner did not pay attention to the tenses s/he used in her/his own sign.
It appears that everyone was respectful to the owner’s wall, if not the sign itself.