While you can't delete a chat in Teams, you can hide it from your chat list or mute it so you stop getting notifications. Here are more details.
Hide a chat
While you can't delete an entire chat, you can hide a chat from your chat list if it's not relevant to you anymore. To hide a chat, select Chat to open the chat list. Next to the chat you want to hide, select More options > Hide.
The chat will be hidden until someone posts a new message to it. (And you can always resume an old conversation that you've hidden as well.)
Note: You can delete individual messages that you've sent.
Unhide a chat
Need to see a hidden chat again? Unhide a chat that you've hidden before by using search.
In the Search box at the top, type a word or phrase that's included in the chat that you want to unhide (such as the sender or a participant's name).
Choose the hidden chat that you want to unhide, and then select Chat in the left toolbar. The chat will appear in the Chat feed again.
In the chat, select More options > Unhide.
Mute a chat
When you mute a chat, you'll still be included in the conversation, but won't get notifications from it.
Select Chat to open the chat list. Find the name of the chat you want to mute. Then, select More options > Mute. If you change your mind, just select it again and unmute it.
An icon appears next to the names of the participants to remind you that the chat has been muted.
Pin a chat
You can pin frequent conversations to the top of your chat list.
Select Chat to open the chat list. Find the name of the conversation you want to pin. Then, select More options > Pin. This ensures that the chat stays at the top of your list. If you change your mind, just select it again and unpin it.
Swipe left on Chat and then tap Mute, Pin, or Hide. If you change your mind, just tap it again to undo it.