Honestly I feel that the ETF2L rule could be adopted and maybe changed a little but it's pretty fair, as long as it's applied properly.
"A team’s active line-up may only consist of a limited amount of higher skilled players that joined after the Provisional Tiers were published. A point system is used to determine this restriction. A player will be given one “point” for each tier of experience higher than the division of the team the player joins.
Example: In 6v6, a Division 2 player joining a Low team will count as two points, which puts the team above the one point limit. For Season 37 experience we will look at matches from Season 34 onwards."
This would mean that the team gets one warning for each match played with this player, and the third warning means permamently losing one league point, in ETF2L at least. This could be changed to be harsher to really put off sandbaggers. I personally played in div 1 a while ago as scout, then when I joined a mid team as soldier was told by admins I couldn't play scout, which was perfectly fair.