If you felt that what they did was wrong then you should report them. Even if nothing comes of it Riot keeps track of this sort of thing and might use it in the future when considering that player's other behavior, or Riot might even use it in the future to calibrate the punishment system for new punishable behaviors.
All that aside, a more direct answer to your question is "no." The person likely won't be punished for it.
Now, that changes a lot under special circumstances. If they admit in chat that they are doing it just to screw you over, then you will have a much stronger case.
If they admit to trolling / trying to do something to hurt the team's ability to win the game then you absolutely can report them and Riot will absolutely punish them. But some people take farm because they genuinely believe it is the best way to win. They can be absolutely wrong about that, but because they are doing it to win they cannot be punished for it.
tldr; if they don't admit to intentionally doing wrong in chat then there's not much you can do about it. If they do admit, then chances are higher that your report will actually work.
You can try asking things like, "why are you doing this?" Because then they might be tempted to reply. If they're doing it for attention, they will probably jump at the chance to try to piss you off. Yeah some people are just garbage like that. If they answer with something like, "to make you lose, get fucked" then it should be fairly easy for Riot to see that the player is indeed griefing / helping the enemy.
To ensure that Riot investigates them directly you can always submit a support ticket explaining what happened. If you do, be sure to not complain about 'stealing farm' until you have made it absolutely clear first that the player admitted to griefing. This will help the support person or the Player Behavior person who handles your case avoid being confused by you complaining about farm being stolen (They probably have copy paste responses for this, because people complain about it incorrectly a lot).