I have had the same problem with an agent on Brockley Rise they put 2 let by signs up about two months ago, phoned them today to ask them to take them down, they said "do you own the property? do you own the road, if the landlord lets us we can leave our boards there for as long as we want". Thinking of taking them down myself and leaving them outside there office! The guy was very charming considering they have hardly any business!