a) I don't really think there is anyone reading this thread now (bear in mind we're 11 pages into it) who hasn't read the book
b) Even if there is, that person would be ridiculously stupid to expect not to come across any spoilers in the course of their reading of this thread.
If people are THAT concerned about spoilers, they should start an alternative thread called "Harry Potter and the Half Blood Prince (NO SPOILERS)" Nowhere in the title of this thread or in the OP does it state there are to be no spoliers.
It's ridiculous to expect no spoliers on this thread. People love to speculate about plotlines and characters, and to expect them to disguise their posts in spoliers all the time is a bit silly IMHO.