You can turn off synchronising of calendars by going into the Your Account settings from the main Navigation Bar at the top right of the screen.
Click on the orange Unlink button to stop further synchronising of your SimpleClinic diary to your external calendar.
When done, the Configure and Unlink buttons will clear.
You can enable calendar synchronising again by clicking on the Link button and completing the instructions in the "Configuring external calendar integration" article.
Click on the Save button to apply the changes.