As part of a carrier obtaining landing rights in a foreign country, it must agree to repatriate any pax who is denied entry into that country. The carrier is also responsible for substantial fines as well as reimbursement for security costs for the pax. Some countries have been known to physically hold the aircraft until the pax is back onboard.

According to the COC of every carrier I've seen, all of these costs are then subject to recoupement from the pax. Cost of fine, security and one-way full Y fare can be enormous. Needless to say, if the pax has a CC with sufficient credit limit, the carrier will see its money, otherwise it's expensive and like trying to get blood from a stone.