This can be changed easily in the Pamela options. Open the main Pamela window and click on Tools - Options - Call recording.

You will see the screen below. Select the top options (red boxes) and click OK. This will make Pamela not prompt you anymore. You may start the call recording at any time manually by right clicking on the tray bar icon (the pink Pamela flower) or double clicking on the Pamela tray icon (which opens the main Pamela window).

Note: Screenshot taken from Pamela Professional. Some options are not available in other Pamela versions.