Usually you can prevent any keys from being entered in a TextBox by Inheriting from a TextBox and overriding the OnKeyPress Method of a TextBox.
Text box to accept only number[
^] there is an example. Any other validation would work more or less the same.
For more advanced TextBoxes you might have to create a
CompositeControl[
^] or a
CustomControl[
^]. This can be quite a difficult and daunting task though.