Deslinde in English means limit or borderline.
The local Guardamar authorities are basically complaining about the Ley de Costas - a national law which controls building within the coastal area.
Central government can overrule local authorities and declare certain properties built within the coastal area illegal and enforce compulsory demolition. This area is limited to a certain distance from the coast - hence the name Deslinde or border.
For more information those with fluent Spanish can search http://www.guardamar.net