It will not happen either each ad counts toward one's annual limit, should someone be able to delete their ad they would than be able to post an unlimited number of ad.
Why not being able to post as many ad as someone want you may ask? Because advertising is not our main purpose/mission therefore there are limits, therefore we must be able to know whether or not a member has reached his limit which is why ads are not deleted.
As for keeping things tidy, no worry that's why the staff is here for.