I hate prawns but I lie that I'm allergic to avoid eating them
Is there a reason why you don't like prawns?
They taste awful. I want to avoid eating something I hate
Why don't you just admit to people that you don't like prawns? No one is going to punish you for being honest so there is no reason for you to lie about being allergic. Lying about this makes you seem childish and immature.
But what if I have a phobia of prawns?
Whatever the reason may be, confess the truth about your food preferences and you won't be forced to eat them any more. You get to choose what you want to eat, so the more you let others know, the more of that type of food will be presented.