Thomas, you are likely looking at the dropdown at the top level of a conversation, which only has "Flag" as an option. The owner of a conversation has more options there. However, right next to every post there is a dropdown that lets you "mute" or "ignore" a person. Muting will prevent that person from posting in your conversations, but it won't affect conversations you don't own. Ignoring a person will collapse their post in any conversation on Cake whether you own it or not.