Newbies are a fact of life. You cannot, you shall not have a game without newbies. A newbieless game is a dying game : no fresh blood, only oldtimers who drop slowly but surely until the game disapears into oblivion.
What IS a problem in Trem is that newbies have a negative effectiveness value on your team. Compare that to an ET game with 5+1 vs 6 ( 5 good players + 1 ewbies ) where the additional noob counts as if it wasn't here and the game is more or less balanced ( 5+0 vs 6 ). In Trem, that noob will be feeding the other team, moving around turrets, etc. It will feel like a 5-1 vs 6 game or a 4 vs 6 which is rather unbalanced.
What can we do to minimise that ? Votekick works but is rather harsh and will transform Trem in a newbieless game which
is a bad thing. Better would be to make changes so that newbies haven't such a negative effect on the game.
Feeders are and will be a problem for a long time. I suggest displaying the remaining kill count for both teams in the score board and not only for your team. A feeder will feel bad seeing how the aliens are getting close to stage 3. It'll have some minor effect on gameplay though but nothing detrimental I say. And it's not like keeping a stage change secret is easy when good players are around tracking the smallest hint of such event.
FF and TK is a problem with such a simple solution : add ETPRO style hitsounds in the game : normal bip for normal hit, high pitched bip for headshot and error style bip for FF damage. Or at least the last one if you don't like hitsounds, I would love them myself.
For newbie builders, it will be harder to mitigate their noobishness. I don't think there is much we can do except make it CLEAR that building is for experienced players only ( like I said already, let's hide the builders when you haven't unselected the tutorial mode for example

). Also, if we take some anti deconner measures, it'll help also to mitigate the effects of a newbie builder deconning the wrong building in the same way it could be used to prevent a deconner from messing with the wrong building.