How do I turn off a repeating trade?

The "Repeat route" button just means that it will perform the last trade route again, not that it will do so into perpetuity. You cannot interrupt the current trade route, but once it ends you will be free to reassign the trader.

Similar question to How do I change or remove a trade route destination?, but the answer is, no, you cannot 'turn it off' or otherwise cancel the trade route.

You have to wait until it runs its course and then reassign it. You can click repeat route again or assign it to somewhere else new.

As the other answers have suggested, selecting "Repeat Route" only repeats the route once, after which you can choose a new destination. Normally you can only reassign after the trade route finishes, but there is an exception: if you declare war on the destination civilization, you can reassign all trade routes going to them and anyone else who declares war as a consequence. Obviously this is a very high cost maneuver, especially just to reassign a trade route you mistakenly sent, but it is still a valid way of reassigning a trade route early if you absolutely must.