Instructions for ignoring people can be found here and covers both normal and hidden profiles. This can also be done on a non-permanent basis if you just want a break from someone.
See the bottom section here…
Keep in mind that people tend to like to switch alts from time to time to circumvent these ignores. As far as I know there isn’t a way to ignore on a per-account basis. I wish this is something Blizzard should address.