I haven't a clue, but I won't refrain from suggesting that it is perhaps a misinterpretation by modern OCR software of the word hexadecision printed in one of the fonts commonly used until recently (well, up to the late 18th century, anyway), in which lower case 's' followed by lower case 'i' is often misread (by the OCR software) as a space followed by upper case 'B' (and when not followed by 'i' lower case 's' may be misread as lower case 'f'). For examples of this try googling "Les contradictions qu il y avoit entre la déci Bon du Concile national de Paris & celle du Concile assemblé par les Légats du sape à Lyon" and observe that while google thinks it's "déci Bon" the book the link to google books takes you to text which is clearly "décision"; or for an English example (also using google's OCR, which is actually better than that used by many academic institutions despite making this error pretty consistently) try googling "acquiesced in the deci- Bon of providence in the fatal contest between the houses".

Hexadecision of course must be the act of rendering null and void (or perhaps just pointless) by magical means the determination of some body or council supposedly empowered to make that determination.

Alternatively, it may be a spell cast by the Mauritius government to improve the island's balance of payments (the spell is embodied at decibon.free.fr). Or perhaps a hex cast upon their efforts by an enemy?

Or, since Decibon is a proper name used both in France and in Sweden (perhaps French immigrants?) hexadecibon may be something to do with using magic to mess up members of the Decibon clan; since you suggested "hexadecibon" was a "relative term", maybe it could be the action of some member of the clan trying to mess up one (or more) of his or her (or its) relatives?

All the above suggested interpretations of hexadecibon are offered without any warranty as to accuracy or even as to credibility. The remarks on OCR errors with certain old fonts are however 100% genuine.