[Wauthan]

As far as I know you can't remove spells or modify research
for one specific nation. But is it really needed? Just don't
give the nation any spellcasters (save priests) or gem income
at all. That way only the pretender and independent mages are
available.

If the player still wants to use spells it's still a possibility,
though a bit more difficult then usual, while it lets you
focus on making a nonmagic nation viable in lategame.
I assume variations of #domsummon might be useful to
simulate new units being constructed rather then summoned.