Yes, you're able switch off specific team member's availability easily.
We call this the "Availability Mode". Please follow the below steps to activate.
Click on the Gear icon to view the Settings page and click on Users & Teams which will be directed to the page shown below.
Next step is to click on the 'Availability' option.
Select the team member, and click 'Online', the team member's availability will automatically be updated and shown as 'Offline'.