Or you could have probed into the real reasons for his question, and gave him the assurance he was looking for.
You don't have to spoil the plot. But if a player fears he's going to stay forever, and that will make him retire his character or leave the game, then "He's staying for as long as I want. Deal with it." might be spoiling a lot more than the plot.if i go around saying -Oh I guess you didnt like him, I'll do something about it next session- Ofcourse he will know the person is for some reason going away next session. The paladin himself isnt a storyarc, and he was a minor character. He stuck with them for -one- session. And I still dont see the big deal on Eric's side, he told me not to spoil anything in the game for him and as such I cannot inform him of when NPC's will come or go. And which NPC's will come and go.
How hard is it to say: "Don't worry, the paladin won't stay for long." or perhaps: "Don't leave or retire your character because of the paladin. He's really not going to be that big a deal. Your current character is fine for this campaign."
You have an extremely black and white view on communication. Answers don't always have to be yes or no. Instead of a short-sighted direct answer, you can also decide to give a real, meaningful response to the issue underlying the question. Communicate!