You want to turn off a CWYW preference from either the endnote word toolbar, EnPref, or the endnote toolbars, Endnote tools, “Cite while you write preferences” - and turn off “open endnote when word is opened”. Then when you want endnote to open, just click on the EN button, and it will open.
Look at some of the other options there and turn them off, or on as you prefer. For example, if you would rather NOT have CWYW as an automatic feature. Once a new document has been created, this has no effect on that specific document, and is only applied to new word documents. Turning it on and off on a previously created document is controlled thru the format bibliography menu.
New users will also want to go thru the Endnote preferences (in Endnote, Edit, Preferences), in particular to set the default library you want to open automatically, and just look at some of the other options available there.