Apart from starting again if they are beyond it, not much. You could try netting them to keep the butterflies away or even a net over the door/windows, provided you don't have any plants that require pollination growing in there too i.e. tomatoes, cucumbers etc. You will need to 'get rid of' the caterpillars already there however you wish and unfortunitely the only sprays you can get for caterpillars has to come into contact with the pest direct and by them time its worked, the little seedlings may not of survived.