You should find the occupier through 'Occupiers' under the 'People' tab in the menu on the left hand side of the screen.

Select the occupier you wish to lock out of the system. 

You can simply click on the green padlock on the right hand side of the screen. This will turn the padlock red to show that they are locked out. When you are ready to give them access to the system again, click on the red padlock and it will turn green once again.