You can't remove the service from within OmniWeb unfortunatley. There are a few third party apps that can help you with this though. Take a look at this:
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http://www.macupdate.com/info.php/id/20192>
Which should be able to do what you need.
You can also go into the Keyboard & Mouse system preference panel, select Keyboard Shortcuts and customize that particular shortcut to something that wont interfere with your workflow.