How to block (or redirect) external image source?
I presume you are allowing the uploading and embedding of images via the WYSIWYG. That being the case you need to write a text filter and apply it to the text format the users are allowed to use (Filtered HTML for example). 
http://local.example.com/admin/config/content/formats/filtered_html

Have a look at the Examples module for how to do that.

filter_example  Defining an input filter


If you're not allowing them to upload and embed via the WYSIWYG (but uploading via a field for example) you could simply remove img from the permitted tags in your Limit allowed HTML tags filter.