I am arbiter myself, but I don't know how to treat player in team matches who try to indicate other players of their team what move to play in position they are in. If they won their game I might first warn them and at the second time lower their score to 1/2 (to 0 in case of a draw), but if they lose their game, there is no way one can punish them. They have literally nothing to lose. According to FIDE rules, they become a spectator one can only exclude from the room. And even in that case, nothing can stop them just to not care and stay anyway. For the refference, we're talking about low contest, like regional once where most of the time the arbiter is actually a player of one team as well.
This is, quite obviously, cheating. I'm going to assume that the teammate isn't asking for help, and the guy is just blurting it out. (Otherwise a forfeit would be a given.) But even if the teammate doesn't want the advice and is doing nothing wrong, it's still extremely unfair to his opponent.
According to the FIDE Anti-Cheating Guidelines:
In a Team event, the team of the offending player shall be excluded from the final rankings. The results for the opposing teams shall remain unchanged. Each of the offender’s games shall be considered a loss, but the score for the opponent shall remain unchanged. All games shall be reported as unplayed.
The prospect of all of the player's games being considered losses and the team being removed from the event ranking is probably enough to deter this behavior. In addition, this counts as a breach of the FIDE Code of Ethics, and the player could be suspended from FIDE events if they are reported to, and found guilty by, the Ethics Committee.
If that still isn't enough to deter them, there's another option. According to the FIDE Laws of Chess 11.3.1, players are forbidden to "use" advice. You could, I believe, inform the teammate that playing the suggested move would be considered "using" the advice and could incur the penalty of a forfeit. Although rule 12.9 does not list forbidding a move as a possible penalty, this restriction is not really intended as a penalty against the teammate (who has not done anything wrong himself at this point) - it's simply an attempt to restore fairness to a game which was interfered with. It's not appropriate in all situations, either.
Prohibiting the suggested move in this situation is specifically contemplated under USCF rule 20E2 regarding unsolicited advice. Other options under this USCF rule include declaring the game a draw, or forfeiting the game on the grounds that advice from a teammate can automatically be presumed solicited.
In a low contest regional type tournament, I'm assuming you don't have to abide by FIDE or USCF rules. Given this, I would suggest the following:
- Warn them the first time you see this happening. Say that the next time you see this anytime in the rest of the tournament, their team loses whatever current match they're playing. So not only does one player lose a game, but their whole team forfeits the entire match.
- Enforce your ultimatum from step 1 if you see them cheating again.
Assuming that the player in the ongoing game did nothing wrong (did not speak to team members about the game) and a spectator team mate just shouts a move, there is unfortunately not much you can do in the moment, apart from removing the spectator from the room. The player did nothing wrong, and you are only the arbiter of the current match, not of the whole competition. The situation is similar to a normal spectator shouting advice.
You mention lowering their score in their own finished game, but in my opinion that is not allowed by the rules -- their game is already finished and the result is already set, they are spectators.
What you can do is have the team leader of the other team lodge a formal complaint with the competition organiser. You as the arbiter can write a factual report of what happened, including the name of the team mate, and send it to the organiser along with the official result forms of the match. They can issue warnings, match losses, a ban for the player, match point deductions, fines, you name it -- you can't. If this sort of thing is consistently reported, they can also know if it is just this one incident or a regular occurance.
If the spectator refuses to leave the playing area after being told to do so, you have bigger problems. I would abort the whole match at that point, write a report to the competition organisation and argue for a 8-0 loss (example) for that team.