how do i close a conversation in imessage without deleting it?

I remember about a year ago, I clicked the x by a conversation on my laptop and i noticed there was a close conversation option. This removed it from being in my conversation log, but it didn't delete the history or the texts. It sort of just hid the conversation, i guess would be a simple way of explaining it. I'd like to do this again now for a conversation with my ex, but I can't figure out how to do this, or if it's even possible anymore since there have been a few updates for Mac since I last did this. How can I do this?