It wouldn't be too bad to create a library function that will do stuff like that - I've created a similar function so that someone can enter many different date formats into the box and it will properly recognize them. In the after_update event for the text box, have something like this:
[TextBox] = Lib.CheckText([TextBox])
Then in a module have the function CheckText look something like:
Public Function CheckText(strString As String) As String
Dim intCount As Integer
intCount = 0
Do Until intCount = Len(strString)
If chr(Mid(strString, intCount, 1)) < ...
.....
Look up an ascii table to find out what character values are invalid, which isn't bad because all the valid ones are basically sequential.