You have to check if the player that enter/exit is the local player.
function bindthemkeys ( thePlayer )
if ( thePlayer == localPlayer ) then
setTimer ( addCommandHandler, 2000, 1, "bb", applybb )
end
end
addEventHandler ( "onClientVehicleEnter", getRootElement() , bindthemkeys )
function unbindthemkeys ( thePlayer )
if ( thePlayer == localPlayer ) then
removeCommandHandler ( "bb" )
end
end
addEventHandler ( "onClientVehicleExit", getRootElement() , unbindthemkeys )