Remove last segment of Request.Url

Much the same as @Oded's answer, but using a UriBuilder instead:

var uri = new Uri("http://www.example.com/admin/users.aspx/deleteUser");
var newSegments = uri.Segments.Take(uri.Segments.Length - 1).ToArray();
newSegments[newSegments.Length-1] = 
    newSegments[newSegments.Length-1].TrimEnd('/');
var ub=new UriBuilder(uri);
ub.Path=string.Concat(newSegments);
//ub.Query=string.Empty;  //maybe?
var newUri=ub.Uri;

Use the Uri class to parse the URI - you can access all the segments using the Segments property and rebuild the URI without the last segment.

var uri = new Uri(myString);

var noLastSegment = string.Format("{0}://{1}", uri.Scheme, uri.Authority);

for(int i = 0; i < uri.Segments.Length - 1; i++)
{
   noLastSegment += uri.Segments[i];
}

noLastSegment = noLastSegment.Trim("/".ToCharArray()); // remove trailing `/`

As an alternative to getting the scheme and host name, as suggested by Dour High Arch in his comment:

var noLastSegment = uri.GetComponents(UriComponents.SchemeAndServer, 
                                      UriFormat.SafeUnescaped);