Например:
For i As Int32 = 1 To 10
Dim frm As New frmNameHere()
frm.Show()
Next
Dim openForms = Application.OpenForms.OfType(Of frmNameHere)()
While openForms.Any()
openForms.First.Close()
End While
Работает также без linq, но тогда вам придется перебирать все OpenForms:
Dim forms As FormCollection = Application.OpenForms
For Each form As Form In forms
If TypeOf form Is frmNameHere Then
'do something with your frmNameHere-Form'
End If
Next