Remove multiple selected items from ASP.Net ListBox using C# and VB.Net

I need to remove a user from multiple selected Active Directory groups in a ListBox (lstMemberOf.SelectedItem.Value).

Below is my code that I use to remove one group at a time for a user. This works fine.  I just can't figure out how to remove more than one group at a time.

Any help would be appreciated!  Thank you!!

Get Groups - Fill Listbox:

Private Sub form1_Load(sender As Object, e As EventArgs) Handles form1.Load
    If Not IsPostBack Then
        Sub getUserGroups()
    End If
End Sub
Sub getUserGroups()
    Dim result As List(Of GroupPrincipal) = New List(Of GroupPrincipal)()
    Dim domainContext As PrincipalContext = New PrincipalContext(ContextType.Domain, "ldap", "username", "password")
    Dim inputUser As UserPrincipal = New UserPrincipal(domainContext)
    inputUser.SamAccountName = lblUN.Text.ToLower()
    Dim adSearcher As PrincipalSearcher = New PrincipalSearcher(inputUser)
    inputUser = CType(adSearcher.FindAll().ElementAt(0), UserPrincipal)
    Dim userGroups = inputUser.GetGroups()
    lstMemberOf.DataSource = userGroups
    lstMemberOf.DataBind()
End Sub

Remove user from selected group:

Protected Sub btnRemoveGroup_Click(sender As Object, e As EventArgs)
    RemoveUserFromGroup(lblUN.Text, lstMemberOf.SelectedItem.Text)
End Sub
Public Sub RemoveUserFromGroup(ByVal userId As String, ByVal groupName As String)
    Try
        Using pc As PrincipalContext = New PrincipalContext(ContextType.Domain, "ldap", "username", "password")
            Dim group As GroupPrincipal = GroupPrincipal.FindByIdentity(pc, groupName)
            Dim user As UserPrincipal = UserPrincipal.FindByIdentity(pc, userId)
            'group.Members.Remove(pc, IdentityType.UserPrincipalName, userId)
            group.Members.Remove(user)
            group.Save()
        End Using
    Catch E As System.DirectoryServices.DirectoryServicesCOMException
    End Try
End Sub

HTML

<asp:Button ID="btnRemoveGroup" runat="server" Text="Remove" OnClick="btnRemoveGroup_Click" />
<asp:ListBox ID="lstMemberOf" runat="server" Width="100%" Rows="10" SelectionMode="Multiple"></asp:ListBox>