Under Spanish law, dual nationality is not recognized for adults.  If a child has a choice between becoming a citizen of Spain, or the nationality of another country, he/she must decide by the age of 18. 

For a foreigner to become a citizen of Spain, he must renounce his former nationality and swear allegiance to the King of Spain.  In doing so, he also swears to recognize and abide by the Spanish constitution and laws.  That said, the UK doesn’t recognize a renunciation of nationality and the U.S. makes it difficult to detach oneself from U.S. citizenship.  It is highly suggested that legal advice be obtained on this topic.