It is NOT safe to remove ANY graffiti.  Please follow this link for a brief list of graffiti symbols:  Graffiti Terminology 

We remove graffiti with products that are AQMD, green and environmentally (VOC) compliant.  Other graffiti removal products may cause injury to workers, vegetation - even those entering the area hours later.  

Removing graffiti can restore up to 20% of Property Value.