The other way (which may even be a tiny bit easier) is to go to the person's profile. Right above their "Last Activity: Today 01:57 PM" line is a pulldown menu marked "User Lists". The options include "Add to Friends List", "Add to Contacts" and... "Add to Ignore List". This way, you don't need to remember the exact spelling of their username just as long as you can find their profile.

You can use that same page to take someone off your Ignore List later on.

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Edited to add: