Graffiti on property that you do not own is classed as a crime (vandalism). See this link.
Below is a gallery of graffiti tags extant in our area. If you know the person responsible, you should report that to police (British Transport Police if it's on railway property). Do you have CCTV and has it shown any culprits?
LCC does its best to have graffiti removed but this is not always successful. At present the City Council does not remove graffiti from its own property and statutory companies are sometimes lax: Virgin Media does not remove graffiti from its street cabinets and Royal Mail and Network Rail are erratic. Scottish Canals has not removed any of the graffiti drawn to its attention. Scottish Power Networks respond promptly as do Openreach. Most private owners usually cooperate and do their best to deal with graffiti but some refuse to cooperate.