I'm using a system which sends out some useless emails and some very important emails. However, they share the same title, so they're grouped into the same conversation by Gmail. I'm trying to apply a filter that adds a "Useless" label to the useless emails. However, this adds the label to the entire conversation. If there are useful and useless messages in the same conversation, how do I make sure that the useless ones don't show up in my inbox?
Agreeing with Al E. above; to my knowledge you will need to turn off conversation mode.
Go to Settings --> "General" tab For the "Conversation view" option, choose "Conversation view off". (See below).
Your messages will then revert to being treated as individual messages. You can then use various methods of folders and/or labels to group your messages as desired (find what works best for you).