Chat messages can be deleted in two ways as discussed below:
Delete message refers to deleting messages one by one in a chat. To delete a message, click on the down arrow besides the message and choose option Delete Message. Refer to the screenshot below -
After choosing Delete Message, you will be asked to confirm if you really want to delete the message as it can not be undone later. This is to insure that some message is not deleted by mistake. Refer to screenshot below -
Clear Converstaion refers to deleting the complete conversation of a window. For example if you want to delete all the messages of one chat room for some reason. It doesn't matter how many messages are there in a room ( few 100s or 10s of 1000s), it will delete all.
To Clear Converstation, choose Clear Conversation Option from Settings Button at the header of a chat window as shown below -
After choosing Clear Conversation, you will be asked to confirm if you really want to clear all the messages as it can not be undone later. This is to insure that conversation is not deleted by mistake. Refer to screenshot below -
Please note that chat messages in rooms can be deleted/cleared only by chat moderators and admins.