What can I do to prevent my elderly father from making internet purchases?

My father who lives on a limited income and is still incharge of his own money (mostly), thinks nothing of clicking on sites and giving his credit card information out, because the site says “free”. Of course, there is always a catch that he doesn’t see, so when the credit card bill comes and I ask him about the charges, he can’t remember or will tell me, “No, it was free.”
I spend hours calling credit card companies and canceling all manner of things. The last thing I cancelled was Travel Accident Insurance. He is 90 and no longer drives. We have already taken away his car, and his mail. He loves exploring on the internet.
I would love to find some kind of control that would not allow him to input his information, or erase it when he tries. some one else mentioned this was possible. I am not very tech savvy but if you could point me in the right direction, I would be very interested in this kind of information.