I suppose, if you really had to, you could place a background image onto the text area and put padding on the area covered by the image so no one can write on it.
However, this is not a clean solution.
What I would do if I was you is use a text field for the uneditable part and set the value to the desired text, e.g.:
<input type='text' value='This text value is hardcoded or generated by a script and the user can't change it' >
Then follow this by a normal text area:
<textarea>type your text here</textarea>