Take a look in the References folder in Solution Explorer, if you're using a web application project instead of a website.  Also look at the Register tags on your pages, user controls, master pages, and skin files.  Next, include all the Register tags and assembly reference tags in the web.config file. 

Most likely, you're referring to two different versions of the WebGridExcelExporter assembly, among all of these places.

You should also look at your licenses.licx file.  If there are any NetAdvantage for .NET control references there, you can remove them - Visual Studio will re-add them if and when it sees fit.  Be sure to not delete the file itself, even if it's empty.