If you look in the ForumSmiley folder, you'll see a file called _metadata.xml. It is read by the forum to load the metadata about the smileys. Smileys will do automatic text substitution (like converting : - @ to angry.gif). But, in _metadata.xml the title for the smiley is set to the string "a n g e l", so any time someone types that word it gets converted.

Change _metadata.xml and delete the entry. This metadata is cached, but for some reason just clearing the cahce wasn't sufficient for the change to take effect. I had to do an IISReset.