If you turn down too many PIA customers, you might not have to worry about it.
Seriously, from your posts here, you seem like a genuine good guy. With that comes the want to help people. As well, that is your business, to help people when they need their cars repaired.
All that said, it is YOUR business and your hands doing the labor. if you don;t want to deal with a PIA repair job, then politely suggest that he could get faster results from another shop or, as Jim suggested, pad the estimate and let the person walk away. No harm , no foul.