There's really no way to do it on a per-character basis. Your only option would be to edit [ Interface > AddOns > nUI > Layouts > Default > nUI_Layout_Default_Dashboard.lua ] and scroll down until you find nUI_DefaultConfig.XPBar -- you'll want to change "enabled = true" to "enabled = false" in both locations. But, that will turn it off for all characters.
What people don't get is that I am, ultimately, an artist at heart. My brush has two colors, 1 and 0, and my canvas is made of silicon.