Set the property to the hosted textbox instead: 
Me.RadSpinEditor1..SpinElement.TextBoxItem.TabStop = False

or create a custom control: 
Public Class MyRadSpinEditor
    Inherits RadSpinEditor

    <DefaultValue(True)>
    Public Overloads Property TabStop As Boolean
        Get

            If Me.SpinElement.TextBoxItem IsNot Nothing Then
                Return Me.SpinElement.TextBoxItem.TabStop
            End If

            Return MyBase.TabStop
        End Get
        Set(ByVal value As Boolean)

            If Me.SpinElement.TextBoxItem IsNot Nothing Then
                MyBase.TabStop = False
                Me.SpinElement.TextBoxItem.TabStop = value
                Return
            End If

            MyBase.TabStop = value
        End Set
    End Property
End Class