How do I turn off the dictation after I send a text on my iPhone?

Dictation is turned on only when you tap the mic.

If you want to disable dication permanently then Settings > General > Keyboard > Enable Dictation [Disable]

iPhone 12 mini, brand new, same problem. The audio symbol is in the chat box and will not go away. It records everything if iMessage is open. Tapping the microphone opens the record box and tapping again closes it but it records anyway.