How can I stop a specific page in the Omega Responsive theme from being responsive
You can use the Context and Delta modules. Delta was developed by the Omega developers and Context+Delta is part of the Omega philosophy.

Use Delta to create a variation on the original theme that does not respond to theme width.  Uncheck "Enable the responsive grid" setting in the theme, for instance. Then use Context to enable your Delta'd theme variation to trigger only on a certain path, in your case the page with the iframe.