Farming kills

Set up a table that logs the number of kills between every two players, e.g. in SQLite3:

Code:

CREATE TABLE kill_parties (
  killer TEXT COLLATE NOCASE, -- killer name
  victim TEXT COLLATE NOCASE, -- victim name
  kills INTEGER,
  PRIMARY KEY(killer, victim)
);

-- upon killing
INSERT OR IGNORE INTO kill_parties (killer, victim, kills) VALUES (:killer, :victim, 0);
UPDATE kill_parties SET kills = kills + 1 WHERE killer = :killer AND victim = :victim;

-- then check the proportion
SELECT (SELECT COUNT(*) FROM kill_parties kp2 WHERE kp2.killer = :killer AND kp2.victim = :victim) / COUNT(*) prop FROM kill_parties kp WHERE kp.killer = :killer;
-- I'm not excellent in SQL, and I'm sure this query can be further optimized.

You may disable rewards if the proportion is greater than a certain value, e.g. 20%. But avoid banning players for it, because maybe the victim just made the killer mad, and the killer is much stronger than the victim, and somehow the victim doesn't want to leave the server.
Or, they two are the most active players on the server and often 1v1 each other.