It is a violation of the game rules to repeatedly attack other players when their percentage of losable buildings (the so called bash level) dropped to zero.
The standard bash levels are 33,% 25,%15% and 5,% but they may be different depending on the server you are playing on. After a certain amount of time (also varies from server to server), the bash level will regenerate, rendering the player attackable again.
One attack at 0% is allowed, since a player may not have realized that his or her opponent has already reached rock bottom. More than one attack at 0% is considered harassment.