My youngster couldn't help but hear my road rage one day as I shouted B*st*rd at someone, of course he latched straight on to it and repeated it several times before i stopped him and told him he had heard it wrong and was using the wrong word. I told him the word I had said was custard and kept correcting him every time he said it, eventually we added the word 'cream' to it and after a while he understood that if something was a 'custard cream' word he wasn't allowed to say it. It worked really well, and even now two years later he says 'custard creams' if he drops something etc!