Only a project administrator can request that his / her own project be removed.
In order to verify the project administrators account, he / she will need to log into Sourceforge.net and go to their project Admin page. Then click on the "Metadata" link on the left menu. Then under "Project Status" there is a link to delete the project.
If your project has moved elsewhere, there also is an option there to mark the project as moved. That often can be better, so visitors & users of your project can find its new location, instead of just getting a Page Not Found.
Documentation: Table of Contents - SourceForge.net Documentation