To deny access to the website or mobile app to one of your customers, go to this customer file.

On Customers → Customers, look for the customer you want, click on "edit" to access its file, then go to "Additional Information" of the left menu and select "User access" tab. 

Once there, just click on the" Remove Access" button.

You'll get a message to confirm the process and it's done. You will still have this person as a customer, but he/she won't be able to access the mobile app or the website until you give him/her access again.