Practice Relating to the Use of Prohibited Weapons
Spain’s LOAC Manual (2007) states: “The use of certain weapons is expressly forbidden. Others can be used under certain conditions.” 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Tomo 1, Publicación OR7–004, (Edición Segunda), Mando de Adiestramiento y Doctrina, Dirección de Doctrina, Orgánica y Materiales, 2 November 2007, § 2.3.b.(3).
Spain’s Military Criminal Code (1985) punishes “any soldier who uses, or orders the use of, means or methods of combat which are prohibited”. 
Spain, Military Criminal Code, 1985, Article 70.
Spain’s Penal Code (1995) punishes “anyone who, during an armed conflict, uses, or orders to be used, methods or means of combat which are prohibited”. 
Spain, Penal Code, 1995, Article 610.
In 2010, in its report to the UN General Assembly on the status of the 1977 Additional Protocols, Spain stated:
Article 85 entitled “Principle of Humanity”, contained in Title IV on Operations [of the Royal Ordinances for the Armed Forces (2009)] clearly embodies the spirit of the [1949] Geneva Convention and its [1977] Additional Protocols, as it provides that “[the] … conduct [of members of the armed forces] in any conflict or military operation must conform to the applicable rules of the international treaties on international humanitarian law to which Spain is a party”.
That is further developed in Chapter VI on Ethics in Operations, which goes into specific duties under international humanitarian law … and prohibits means and methods of warfare not in conformity with international humanitarian law. 
Spain, Report on the Status of the Protocols Additional to the Geneva Conventions of 1949 and relating to the protection of victims of armed conflict, 5 May 2010, Section 2.