There isn't a setting to control the formats of the export. It's either all there or none.

What you can do is more a UI trick to hide the Excel 2007 option. By using some JS/CSS you can hide it. Based on the code you can make it work on a single analysis or on the whole browser page (= dashboard page). You add the code to all the dashboard pages and you have your "dashboard level" control.

Use your browser developer toolbar to identify how to uniquely select that entry of the export options.
You have jQuery loaded by default in 12c, therefore you can use to precisely select that entry and either remove it from the DOM or hide it. Just remember to add the even handler to run that code only when the document is ready, otherwise it could be executed too early and generate errors.

More in generally I always advise against adding custom JS/CSS code in OBIEE if it isn't something you master, because you will simply lose control on it, what it does and how. And at every update/upgrade you need to maintain that code because the OBIEE pages code can change and that JS/CSS is custom code you are in charge of (and not something Oracle has to care about).