Consider this example: user tom is being banned from booking …
You can put following code in your theme functions.php (if you are lazy) or better create a plugin with all your rules
function my_em_mods($result,$EM_Booking){
global $current_user;
get_currentuserinfo();
if ($current_user->user_login == 'tom' ) {
$EM_Booking-> errors = array_merge($EM_Booking-> errors, array ('tom cannot book'));
$result = false ;
}
return $result;
}
add_filter( 'em_booking_validate', 'my_em_mods' ,10,2);
For your example, you will have to use a meta for your users that will hold the current booking info