Never did need the excuse of having a drink or whatever lame excuse to refuse an extra duty, especially operating back.
I know one person who arrived at the airport as a passenger and the ASM realised who he was and then took him to one side and explained that his services were being requested to operate back. If you pull the 'I have just had a drink' card then that closes the matter there and then. You will spend a lot more time getting them to listen if you simply just say 'No'.