How can you block Cox Cable from taking screen shots of your computer screen?

My housemate just told me Cox Cable told him that they take screen caps of our computer screens. How can I block them from doing this? They can observe me reading my email, checking my bank activity, whatever I’m doing on screen, they could possible “see.” I’m not ok with this! What are the methods by which I can block them, while still being connected. I’m wireless and on a Mac. Please, if you can, provide both Mac/PC remedies. Thanks!