Assuming that Krypton27 is a citizen of an EU country, he/she needs to learn about his/her rights.
EU law requires all EU member states to admit any EU citizen (who can prove their citizenship by producing a current passport or national ID card). So Spain, along with all other EU countries is legally obliged to admit any British passport holder whose passport is still valid (even if there are only a few seconds left before it expires).
While the Canary Islands are regarded as a non-EU country for the purposes of bringing back items such as tobacco (because the islands have their own economic and fiscal regime), they are actually within the EU and the same immigration laws apply as in Spain.
Confirmation can be found on the websites of the Canary Islands Government
http://www.turismodecanarias.com/en/sobre-cana rias
and of our own Foreign & Commonwealth Office
http://www.fco.gov.uk/en/travelling-and-living -overseas/travel-advice-by-country/europe/spai n?ta=entryRequirements&pg=4
Chris