As it stands right now, you cannot black list people who have sent you friend invites even if you have not accepted the friend invite.

Also, when you click "no" on the invite, it doesn't go away. It remains pending on your friend list for about 3 minutes.

Gilsellers on adamantoise have been abusing this for the last week. They send you a tell advertising their website and then immediately send you a friend request. The process to blacklist now becomes:

Go to friend list. Find their name. Decline friend invite. Right click on name in chat. Blacklist.

I know it doesn't seem like a big deal but it has been happening to me all morning and it's starting to drive me insane. I miss the days when we could play MMORPGs unharassed by currency sellers.