Yes, you may switch off specific call representative's availability by turning off their availability.
We call this the "Busy Mode". Please follow the below steps to activate the Busy Mode.
Click on the Gear icon to view the Settings page and click on Platform which will be directed to the page shown below on the screen shot.
Next step is to click on the Calls option from the ribbon list mentioned on the page.
Scroll down on the page and click on the arrow button on the Call Features options.
Now you can turn the button to ON for Busy Situation as shown in the screenshot below and click on Save.