When an email topic is no longer required, only the sender has the ability to close it. Closing topics are category-specific, if there are multiple categories part of the conversation, closing one topic will not impact another if it's from a different category.
From Home, select Inbox.
Select the email that you wish to close.
Tap the ellipsis "..."
Select Close Topic.
You will need to enter a reason in the Close response, and tap Close.
A success prompt will appear.
The green tick indicates that the email has been closed, and the Closed status will appear in the email.