Unsporting trades in the end-game of settlers of Catan
This situation is called The Kingmaker Scenario.  It is an effect that is largely seen as being undesirable in games, yet at the same time is also considered somewhat inevitable in games with 3 or more sides in a Free-For-All situation to at least some extent.  Game designers are always trying to find ways to fight against this effect, and there are a number of ways to reduce the effect that Kingmaker has on the game when designing a game - here are a few examples:


Completely eliminate a player from the game after they are behind by a certain threshold
Create a "comeback mechanic" that allows a player to fight back against the feedback loop holding them in a low position
Using concealed information to make it more difficult to determine the values involved in the victory conditions


When dealing with a pre-made game like Settlers of Catan and not with a game of your own creation, your options are limited - you can either create house rules of some form, or simply accept that aspect of the game.  It may be best to try and find what others have done to solve the same situation for the same game - if it is widely considered to be a problem by other players, than there may be popular and well-tested house rules that already exist.  On the other hand, if there are not, it may be a sign that regular players do not consider it to be a major factor in play.