By definition, the ones who declare that they are carrying a bomb are not Jihadists (i.e. obviously don't want to die) So straight away you can say that they lack a certain dedication or conviction and are likely joking or just plain whimping out.
Either way, me thinks a swift and merciless public beating may be in order, delivered by none other than their fellow passengers, whom after years of dreary commuting, dealing with airport security and exhorbitant airport food prices are barely able to contain their pent up rage with the world. If t'was such an occurence on my flight the anarchist in me would love to p.a:
"Ladies & gentlemen t'is the cpt speaking the guy in seat XX has just threatened your lives with a bomb. The cabin crew are just now distributing handcuffs and rolled up newspapers so that you may conduct your own anger management thereapy. This free therapy session was brought to you by xxxxxx airlines." Followed by something apropriately creapy from the old testament "And you will Know my name is Joe when I lay my fingers upon thee."
Conversely:
Maybe rather than wait for someone to joke / admit they have a bomb; maybe security could just have guns levelled at all who enter the sacred "sterile area". The solution would be for security to ask everybody if they are carrying a bomb.
"Excuse me sir do you have any aerosols"
"no"
"Do you have laptop in your bag"
"Um No"
"Metal in your shoes"
"Uh..No"
"More than one lighter"
"Nope"
"Are you carrying a ticking time bomb"
"Yeh... ok you got me"
BANG BANG RAT-A-TAT TAT.
"Excuse me Madam, do you have any aerosols"......