It's on their profile page. For example if you wish to put me on ignore click on my username and select my Public profile page, then it is on the right hand side of the blue band under my username. I think there may be another way to select it too.
I hate using the ignore feature because I haven't met anybody here that I can't learn something from, even if it is how to sit on my hands!
You can also add someone to your Ignore (or Buddy) list by clicking on "User CP" from the left side of the blue bar near the top of the page, then click on "Buddy/Ignore Lists" near the bottom of the list on the left side of the next screen, under Misc.
You can also access your User CP on the left side of the screen when you have your Private Message center open.