Zappo
Explorer
Ok, first of all I'll go against the flow and suggest that you lower the intra-party conflict level. There's good intra-party conflict and bad intra-party conflict, and the note-passing, poison-preparing one is the bad one. I'd forbid note-passing as Mark suggested; that could be enough to get the point across.
While the backstabbing phase often just passes, and other times the players take it as just fun, I've seen a campaign die because of it; it's not a pretty sight.
The peeking behind the screen must be stopped immediately. Having the player sit at the opposite side of the table should be enough.
As for the burning of the oasis, I don't necessarily see it as a symptom of bad roleplaying. The plant life was hostile, and the insects were dangerous, making the oasis itself useless to anyone: the party couldn't even reach the water. Burning it is a drastic solution, but not an insane one. Now the water can be actually accessed (depends on how contaminated it has become, though), and the plants will eventually grow back. If someone tended to them, they could be useful plants instead of poisonous monsters.
I don't suggest docking XP for "bad roleplaying" or anything else. Use awards for good roleplaying, that's enough.
While the backstabbing phase often just passes, and other times the players take it as just fun, I've seen a campaign die because of it; it's not a pretty sight.
The peeking behind the screen must be stopped immediately. Having the player sit at the opposite side of the table should be enough.

As for the burning of the oasis, I don't necessarily see it as a symptom of bad roleplaying. The plant life was hostile, and the insects were dangerous, making the oasis itself useless to anyone: the party couldn't even reach the water. Burning it is a drastic solution, but not an insane one. Now the water can be actually accessed (depends on how contaminated it has become, though), and the plants will eventually grow back. If someone tended to them, they could be useful plants instead of poisonous monsters.
I don't suggest docking XP for "bad roleplaying" or anything else. Use awards for good roleplaying, that's enough.