I have always thought that a team (ie, Freeps or DS) should be able to refuse a player to their 'side' after a certain number of turns.
Maybe a better way to eliminate the late declaring neutrals from jumping to the winning side would be to have a stipulation, for the PRS at least, that neutrals have to have been on the winning side for 50% of the turns run, and increase that percentage after 20 or 25 turns and every 5 turns after that. Any Neutral that switches before turn 5 would be waived this requirement.
I do not get into that many games usually only 1 at a time with some overlap. Some of the players I don't know, some I do. The players that I have played with/against I have an opinion as to whether they are team players or not and if they are good players. It would be fairly easy to deal, in game, with the "cunning" players on your own team to make sure that they do not do well. On the other hand if that "cunning" player does not help support the team, than the team does not win and their own ratings suffer.
Sorry if I have gone over points already covered.
Steve P