you mean you've rounded the allen key fitting on the top??
If so, try a flat headed screw driver wedged in and tapped (gently) with a hammer to get a tight fit, then use mole grips on the screwdriver.
Which banjo fitting is it? If its the one into the reg then its gonna be tricky, if its the others then surely a pair of mole grips will do it.
-ChEf
If ya knacker ya banjo's I got a spares set F/S