You could try using the On Key Press command to check if the key pressed is a numeric key or not, like this:
If Not IsNumeric(Me.TextBox1) Then
'code to delete the Text entry
End If
but i've had problems with this, as editing the Field highlights the entire entry which means that every time a key is pressed it highlights the current text, so that when you press another key it overrides it, (if that makes sense!)
Good luck with it.
hope it helps,
muzz