it's not an ignore function, but there's a particular Avatar that irritates me so just now I decided to see if I could single out and block it with AdBlockPLus (a plug-in for firefox that removes most advertisements from websites). fortunately the avatar is stored offsite so I just added the address of the avatar to the filter list from the preferences menu in Adblock, and when I restarted my browser, Voila! gone.
just realized even avatars uploaded to the Flag are going to have an address you can block, plus any sig's that are huge and wasting your page loading speed.