The problem is that it is basically impossible to completely close and remove a software project from Sourceforge. The best you can do is tag it as "inactive" or "relocated" and provide a link to the new site, but the project site will still exist.
Let's say it becomes possible. Then what? If your project is popular, it would be on softpedia, filehippo, majorgeeks and many other "free software" aggregators. If your project license allows others to distribute your software, you can't stop them.
