Look for the injector bypass hose running from the number one injector to a banjo fitting on top of the secondary fuel filter assembly. Remove the bolt which secures the fitting and replace it with a standard bolt - i.e. not hollow with hole punched in the side. I think this'll do the trick.

Jay.

P.S. Forgive my ignorance, but why do you wish to stop the return flow?

__________________
On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er