It's all well and good to have clauses in the T&Cs but implementing it is another story. Believe me I know.
Another thing you may not realise is that bans are very rarely for life, so banning a nervous flyer for 6 months probably wouldn't really achieve much.
Put yourself in the place of an airline employee. Do you think we want to be inconvenienced like this. Do you think a crew member wants their lucrative BKKSYD jeapordised if they run out of hours on the ground sorting out situations like this?
I really wish passengers who post on these forums claiming 'airlines should do this' and 'airlines should do that' would actually listen to the responses given instead of arguing the toss and in a lot of cases putting us and our career choices down. You have plenty of opportunities to make our lives miserable when you fly with us. Why invade our spare time as well?
Bottom line, airlines don't ban people in situation like this because they can't. Accept it.