EXP Decay
Suggested by Arius, EXP Decay reduces the experience points yielded by enemies upon death the more they are killed. Eventually a certain threshold is reached, set to whatever the developer desires. It’s very possible to reduce the EXP gained to zero after a certain number of kills. I like to call that a rough anti-grinding technique but if used badly, the results will be unfortunate.
Again, screenshots aren’t necessary.
Instructions
Notetags are used to make EXP Decay work, and they are used on the enemies. Three different tags are available:
<no_exp_decay>
By default, all enemies are affected. This tag prevents any decay to be applied on the enemy.
<decay_rate: x>
The experience yielded will be reduced by x% per kill.
<decay_max: x>
The experience reduction will never go past x%.
Default values
In the script config section, you will find two default values for rate and maximum reduction.
DEFAULT_EXP_DECAY_RATE = 25
DEFAULT_EXP_DECAY_MAX = 100
Change them at your leisure in order to avoid having to use too many tags.
Grimoire Castle 
What about EXP decay by actor level? So you gain like 15 EXP from a blob on level 1 and then only 13 EXP on level 3. Some people might know this kind of mechanic from Paper Mario.