[@KatherinWinter] Why I am quite flattered madame! ^___^

Eh...depends on the level of Merlin's Magick (which i'm assuming is beyond comparison) perhaps he could use some sort of mind trick that causes Evelyn to relive her darkest memories every time she disobeys or it could even be a curse that causes Evelyn horrendous amounts of pain each time she lashes out angrily at someone, since anger is her greatest issue and what she derives her power from.  Not the best ideas, but just what I can think of off the top of my head.