I'm not interested in any of them and would like to replace them with my own fave streams. Is there a way to delete/remove them? I can't even find them in the external DB.

The answer is simple: you can't as the Cool Streams are a selection made by the Amarok developers.

You can write your own script for a stream collection instead, have a look at the already available scripts for Amarok 2.x on http://kde-apps.org