From APP
1.Please Swipe left to label conversations and choose "more", then you will see Delete/Archive/Block/Mute button to manage your conversations, if you want to unblock/unmute them, you can operate here as well:
2.Click into the chatting page, then click the profile as below picture, then you will see Add notes/Pin to top function.
From PC
You could go to [My account] - [Message], then click to the right of the conversation as below, you will see Delete/Mute/Block/Archive function, if you want to unblock/unmute them, you can operate here as well:
Please note that the deleted messages will be back again if you there is new messages from the same supplier.