How do you reply to “is this an issue?” and stay professional?

I need some advice for an email response. I've had a higher up (but not my boss) been rude to me and others for a long time. I've had enough of it. I've tried ignoring and replying with kindness and it continues. I need to reply sternly but professionally. Any ideas to responding to “Is there an issue with that?” when they make a request?