Click their Forum avatar and choose View Activity. Then click the icon at the top right that says NORMAL. That will give you options to leave as is (Normal) or to Mute or even Ignore.
In the event that they have their Forum Activity hidden, you can add them to your ignore list via your Activity page.
Click your Forum avatar and choose View Activity. Then click Preferences, then Users to go to the screen where you can manually add Forums Users to your Ignore or Mute List."
That solves my question.
Have a nice day.
#Edits: I could not make a new post. So I have rebuilt this one to match solution.