By-laws governing the city allow the municipality to recover costs for the removal of posters from the individuals or organisations hosting the advertised event - but even though this is the case, I have my doubts that this is often enforced (probably because there's so many other things to take care of). Regulations regarding posters can be found in Section 11 of the December 2001 Western Cape government gazette. It's probably a good idea to review the document if you're in the event industry. ;)
Stefan Jansson
I don’t think people ever remove their old posters.