It's really very simple everybody.
Carrot and stick.
Buy off the Basque elite, create some government pork barrel jobs, and then crack down heavy on everybody who assists Basque seperatism activities; from pay offs to local Basque thugs, to "political" activites, to stopping trade of "those" regions and or people who don't comply.

The Basques are more Spanish than most of the French, the Portugese, and for sure the rest of the Europeans.

Basque seperatism is merely a little left over vomit from the Franco era.

p.s. words to the Spanish National Anthem wouldn't hurt either.