Basic Notion & Motive
Some players cannot resist the call of the grinder and tend to build large scale grinding structures in which they collect the debry of fallen Barrow-wights. This provides them valuable loot for a relatively (too) small effort. This was touched upon in this thread, and Direranger50 pointed out the possibility to alter the spawning conditions for Barrow-wights to ensure they cannot spawn in player generated grinding traps / mob farms. Main reason for the change is that I believe the mod is designed to be an adventure rp survival game and any possibility for players to grind/farm for resources through exploiting the base minecraft vanilla mechanics should be discouraged or disabled. As I prefer simple solutions, I pondered a simplest possible option after checking out the present spawning behaviour.
Proposal
Make Barrow-wights spawn on podzol only. This is a block that is already available in the game and generates in the Barrow-downs resulting in sufficiently significant occurence.
With the mechanics present in the mod, and no access to enchanting, Silk Touch enchantment is not available to players and podzol is not craftable and cannot be relocated by players, which makes it impossible to create large spaces where large numbers of Barrow-wights would spawn.
Feature mechanics
As the occurence of Podzol is relatively low in the Barrow-downs, it is advised to increase the base spawn rate by either increasing the LOTRspawnEntry weight factor, or increasing the spawnCountMultiplier for the biome. I would guess the first would give the best results.
Alternative options
1) As equally effective alternative, you could consider to create a special block that only generates in the Barrow-downs, which is not renewable or relocatable by players and that is the precondition for Barrow-wights to spawn on, as proposed by Direranger50 in this post.
2) Another alternative would be to change the spawning mechanics of all NPCs and make them recognize player made grinders. That would be quite a different approach and is beyond the scope of this proposal, so I prefer to not go into any further detail on that option here.
3) A third alternative was suggested by Drago and concerns making wights immune to fall damage. This would of course effective to counter grinders that rely on spawned entities to fall of high platforms and die, but will not be effective in other types of grinders, where players can hit and grind trapped NPCs without them hitting the player. As this change would be justifiable for reason of wights being able to hover, this change may be considered regardless the anti-grinding purpose of this thread.
Related subjects
As grinding may also be a matter of creating a structure in which players can target naturally spawned NPCs while ensuring those NPCs cannot hit them, it may also be worthwhile to consider to make NPCs recognize such situations and adjust their behaviour accordingly. This option has already been discussed in this proposal:
[Endorsed] Decrease chances of NPCs running into grinding traps
Here's a few other interesting, but less related suggestions I found in my (incomplete) administration:
[Endorsed] Angmar related stuff.