Roo has advice in his signature, thus:
If you wish to add a troll to your ignore list, click "Profile" then "Buddies/Ignore List".
This is correct, and yet twice I've failed to understand it. Learned how and then forgot and had to learn again. Possibly it's just me. But the ignore feature is strangely hidden: you have to execute exactly the right sequence.
My first mistake, both times, was to interpret "Profile" as meaning the profile of the intended ignoree, rather than my own profile. This initial error was reinforced by the fact that you can "Add to buddy list" after clicking on another member's profile, but cannot ignore him there.
So, you click on your OWN profile (at the top) then select "Account Settings", then on that page "Modify Profile" button and click on "Buddies/Ignore List" from the drop-down. Here we come to my second failure: It defaults to Buddies and highlights it, and then the eye naturally moves down and sees only Buddy-related text and buttons and whatnot. But up beside the bright orange "Edit Buddies" there is in fact a hiding-in-plain-sight text "Edit Ignore List", which turns into a button if you mouse over it, which I didn't do initially.
The better option is to mouse over the last drop down option of "Modify Profile" and let it show you both options and choose from there.