disabling 'retrieved from' url for APA 6th edition

I'm working on a large thesis with hundreds of references. I've used google books/ref man to download citations into zotero and this is automatically reflected in the url field. However in many cases I've also read a hard copy of the book and therefore (according to APA 6th ed) don't need to include this 'retrieved from' url.

Also some entries were imported from endnote and zotero has automaitically used the url field to record reference to where the original file was stored on my hard drive. This is information I definitely don't need to include!

How can I disable this default function rather than editing every entry?
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