How can a `TextBox` be prevented from being edited before a given position?

For example, if a `TextBox` contains the string:

> 
> Example Text: The black cat.
> 
> 
> 

How can I prevent the user from editing anything before `"The"`?

I can try to trap the `Backspace` key with the `KeyPress` event, but how can I use `MouseClick` to prevent the user from moving the cursor to a position before `"The"`.