Bring a large pot of lightly salted water to a boil. Add pasta and cook for 4 minutes or until al dente; drain.
Melt butter in a large heavy frypan over medium heat. Saute prawns for 1 minute on each side. Stir in garlic and cook for 1 minute. Remove prawns with a slotted spoon; set aside. Stir in flour and Cajun seasoning. Cook, stirring for 5 minutes. Gradually whisk in milk then cook until thickened. Remove from heat and season with salt and lemon juice. Return prawns to sauce and spoon over cooked pasta.