EndNote does not have the ability to delete all citations in one command.
You can use Edit and Manage Citations to remove them one at a time or use the option to Convert to Plain text which would unlik the citations from EndNote but they still would need to be removed from the text one at a time.
You can find those different steps in our Knowledgebase:
If you would like to see this feature in a future version of EndNote, You may want to submit a request for a product enhancement on our website.
http://community.thomsonreuters.com/t5/EndNote-Product-Suggestions/bd-p/en-suggest