Disabling pasting of images

Today, while working on a customer’s blog I noticed that the homepage was taking quite a long time (>30 secs) to load.

I assumed my customer did upload a large image, so I decided to investigate.

When I looked at the code I found a few loooooong string like this:
[Code moderated as per the Forum Rules. Please use the pastebin]
zillions of characters follow.

When asked about this, my customer admitted he simply copy-pasted the image into the visual editor.

“Yes it works! It’s so simple! It’s cool!” he said

“ARGHHHH It’s disasterous!” I said. The performance of the pages has degraded horribly.

Now the question is: how do I disable this feature?

Any help is very appreciated!