Bring an end to estate agents' free advertising

We live in a country where housing sales are dictated by estate agents whose sole objective is to overstate the real value of homes in order to increase there own profits, one of the very useful rights they have which no other small business is entitled is free advertising this gives them the right to to display boards outside properties for let or sale which can then remain in theory for two weeks after sold or let as i am sure most people are aware some stay up for months after, in this age of internet, newspaper advertising is it really necessary for streets to be blighted by these boards.