You should contact the local council for the area where the graffiti or fly posting has taken place.
If the person/organisation is known, then it may be possible for the council to issue penalty notices and a 'defacement removal notice' (in Wales this is a 'graffiti removal notice' and applies only to graffiti).
If it is on council-owned property, or if it is offensive/racially abusive, the council may be responsible for removing it. If not, then the council may be able to advise you who to contact or remove the graffiti at a charge.
See the website in Related Information to find out your local council's details.