You can long-hold on a specific message that you have sent and you will have an option to delete either your own message, or any message if you are an admin in the team.
You also have the option to achieve a full conversation to declutter or and clean up in Heja. This only achieves the conversation for yourself and is done by long-hold on a single conversion in the list of all your chat conversations.
You can also leave a conversation if you no longer want to take part of it. Click on the conversation -> select the icon in the top right corner of your screen -> and choose "Leave conversation" 💃