I gave him something else to focus on. Or rather, someone. Kefka didn't exactly make it difficult to do so, he was quite obviously spoiling for a fight, and even attacked Tartaros' former headquarters in Everglade. Eventually, his instincts as an Etherious took over, and he recognized Kefka as the greater threat, not just to me, but to himself as well. Ultimately, his natural inclination to serve my will won out over his pride, even if I expect he would have immediately focused on Fairy Tail again afterward if given the chance.

{ It also helped that Kefka expressed more than a little interest in E.N.D.'s book, and Zeref gave Mard Geer explicit orders that Kefka was not to lay one finger on it, even at cost to Mard Geer's own life. }