In IIS 7.0 integrated mode after deleting all headers with Response.ClearHeaders() IIS would add some other headers like Server
You can add this to your Web.Config:
Update: if you're using the MVC framework I would also recommend removing the X-AspNetMvc-Version and X-AspNet-Version headers as well. This is accomplished by setting MvcHandler.DisableMvcResponseHeader = true in your Global.asax file and in your Web.config respectively.