Aquí tienes una posibilidad:
Function FuncRTrim(ByVal strParam As String) As String
Dim pos As Integer
Dim espacioBlanco As Boolean
pos = Len(strParam)
espacioBlanco = True
While (pos >= 1) And espacioBlanco
If Mid$(strParam, pos, 1) = " " Then
pos = pos - 1
Else
espacioBlanco = False
End If
Wend
FuncRTrim = Mid(strParam, 1, pos)
End Function