I guess some people just lack the civic sense or may be completely oblivious to how bad they are stinking. If spotted by ground staff they can be asked to shower before they board the plane. It can even be printed on tickets as for boarding time to shower before boarding so as not to offend other passengers. If even after this they stink for no medical reasons, guess they should be denied boarding. It sure is a torture with someone smelling bad even for a split second what to talk about even the shortest of the flights to tolerate their stink.