Yann,

What about the ability for you to lock a thread? 
Some threads get out of hand and really need to 
be stopped from accepting new additions so that 
everyone can calm down for a bit. While this does 
not stop people from posting a new thread on the 
same subject, it would make them look like they 
want to pick a fight if they do, especially if 
people have asked for everyone to step back for a 
moment and gather their composure. Moreover, the 
intensity decreases in the new thread more often 
than not mainly because less people participate.

How is this different from you deleting a thread? 
When you delete a thread, no one else can decide 
for themselves what happened and has to take the 
words of others or judge from what they last 
read. On the other hand, a locked thread is still 
readable to all.

---Trebuchet