Through BT Pay or NeoBT.

Through BT Pay:

1. Go to "Card details"

2. "Card settings"

3. "Save with Round-Up"

4. "Disable Round-Up"

By NeoBT:

1. From the "Products" menu

2. "Round-Up"

3. "Close Round-Up"

It's good to know that you can reactivate the Round-Up whenever you want. 🥰