When killing infernalmobs and achievements disabled in the config I am still getting the achievement message:
Special Kill Awarded! MobHunting
1/100

There are more ways to disable rewards for achievements? How did you do it?

please enable debug information using /mh debug , make a kill where you get the reward and let me see the Mobhunting information from the server log?

This is very odd. I've killed a few infernalmobs with a sword and the message didn't pop but when I killed one with a bow and arrow the message popped. It doesn't always cause the message to show up, I think I have to wait a bit or kill a different type of mob. As for how I disabled I only put the world under disable-achievements-in-worlds.

Here's the debug message I got from the kill with the message:
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] ======================== New kill ==========================�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Bladen killed a Infernal Spider (InfernalMobs)�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Permission 'mobhunting.mobs.*' or 'mobhunting.mobs.SPIDER' not set, defaulting to True.�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Basic Prize=6 Rupees for killing a Infernal Spider�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Checking if player is grinding within a range of 15.0 blocks�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] DampendKills=7�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Bladen has no Rank Multiplier�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Multiplier: �[0;36;1mInfernal = 1.25�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Multiplier: Difficulty = 0.75�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Killstreak=14, level=1, multiplier=1.0 �[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] [AchievementBlocked] Achievements is disabled in this world�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] RecordAchievement: hunting-level1-spider has 2 kills�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] RecordAchievement: infernalmobs-hunting-level1-spider has 1 kills�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] RecordKill: Bladen killed a Infernal Spider (InfernalMobs) Cash=5 Rupees�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Bladen got a reward (5 Rupees)�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] Command will be run if chance: 0.05 > 0.1218806267853263 (random number)�[m
[21:28:56] [Server thread/INFO]: [MobHunting][Debug] ======================= kill ended (37)=====================�[m

I think this is fixed in build 437 or newer
htttps://fractal.lindegaard.one:8181/job/MobHunting/

Please test and let me know?