A few months ago when I was giving a presentation on Reporting Services someone asked me how to restrict the selection of export formats. Specifically, they wanted to remove XML and CSV formats since the output is not very user friendly and it's only useful in very specific scenarios. At the time I didn't know the answer but I found out since then.
The items in the Export dropdown list can be controlled through the RSReportServer.config file, that's the RS config file stored in the ReportServer folder. First, find the <Render> section. In that section you should see a few rendering extensions, it's pretty easy to figure out what rendering output they map to. Once you find the format you want to hide, add this attribute to the <Extension> node:
Visible="false"
You should be able to see the Visible attribute on the NULL extension, the other extensions probably won't have it because the default value for Visible is true. Once you make the modification, the extension you modified should no longer show in the Export dropdown list. This modification only hides the export option in Report Manager but it still allows exporting into that format either programmatically or through direct URL access. So, you wouldn't really prevent savy users from modifying the URL of another exported report into something like "...Format=XML" and exporting into XML. If you want to disable let's say the XML format all together, comment out the the entire <Extension Name="XML"...> node to un-register the XML rendering extension.