filtered textbox extender question? add numbers

hi,
i am using the below but want to prevent user input of numbers as well?

<cc1:FilteredTextBoxExtender ID="TextBox1_FilteredTextBoxExtender"             runat="server" Enabled="True" TargetControlID="TextBox1" FilterMode="InvalidChars" FilterType="Custom" InvalidChars="~!@#$%^&*()_+[]{}|\<>/?" >         </cc1:FilteredTextBoxExtender> the code below but want to prevent input of numbers as well ?
Is it possible also to warn the user with a label or message popup if they try to enter invalid values>