Mix together the Cajun seasoning, lime juice and vegetable oil in a resealable plastic bag. Add the prawns, coat with the marinade, squeeze out excess air and seal the bag. Marinate in the refrigerator for 20 minutes.
Preheat a barbecue for medium heat and lightly oil the grate. Remove the prawns from the marinade and shake off excess. Discard the remaining marinade.
Cook the prawns on the preheated barbecue until they are bright pink on the outside and the meat is no longer transparent in the centre, about 2 minutes per side.